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23.

There are two outs, the bases are empty and the batter is 23.

He is, as he steps into the batter's box, the most exciting player in baseball. As a rookie, he has hit a ball 513 feet in batting practice, thrown a pitch 102.5 mph from the mound and reached a top sprint speed -- nearly 30 feet per second -- faster than three-quarters of his peers can touch. These are the highlight years, and it feels like just the beginning.

But an athlete's physical decline begins before most of us notice it, and even the 23-year-old body can do things today that it might not be able to do tomorrow. Fastball speed starts going down in a player's early 20s, and spin rate drops with it. Exit velocity begins to decline at 23 or 24. An average runner slows a little more than 1 inch per second every year, beginning pretty much immediately upon his debut. It takes a little over four seconds for most runners to reach first base, which means with each birthday, it's as if the bases were pulled 4 inches farther apart. Triples peak in a player's early 20s, as does batting average on balls put into play. A 23-year-old in the majors is twice as likely to play center field as left field; by 33, the opposite is true.

Thirty-three feels so far away, but it's already happening. The 23-year-old's lean body mass peaked sometime in the preceding five years. His bone-mineral density too. He's at the age when the body begins producing less testosterone and growth hormone. His body, knowing it won't need to build any more bone, will produce less energy. Male fertility peaks in the early 20s, the same time as pitch speed and exit velocity. Athleticism is, crudely speaking, about showcasing what a body looks like when it's ready to propagate a species. The 23-year-old's machine works as it was designed to. It is undamaged, unsmudged, and every circuit in it is trained to carry on his family's tradition of survival. When you're 23, the 32-year-old Mark Trumbo says wistfully, "performance is the only thing holding you back." To watch a 23-year-old athlete is to see the perfect machine running perfectly.

The batter grounds a pitch back up the middle, inches past the reach of the lunging infielders. It appears to be a routine single, but in the fraction of a second that the center fielder leans back on his heels to gather it, Shohei Ohtani tears around first and slides into second base for a breathtaking hustle double. His team is ahead by six runs in the ninth inning. It is something that only a 23-year-old could do, and that only a 23-year-old would want to do.

Two weeks later, Ohtani's season is interrupted by a sprain in his elbow ligament. The perfect machine has broken. They will try to fix it, and he will certainly return, but when he does, you can expect him to run an inch slower per second. They stop being young sooner than you think.

26.

It's late May, and the runner on third base is 26.

He's the best player in baseball, but he has, technically speaking, lost a step: When he was a 20-year-old rookie, he might have been the fastest runner in the sport. Now he's merely fast. As a rookie, he made four home-run-robbing catches; now, at 26, he hasn't made one in almost a season and a half. Yet he has not yet begun to decline as a baseball player. He's having, by most measures, the best season of his career, and he's the easy front-runner for American League MVP. It's an odd quirk of aging patterns that ability declines before performance does: Exit velocity declines years before home runs do; speed declines years before stolen bases do. Bone density might peak around 20, but ballplayers, most aging curve studies have concluded, peak in their mid- to late 20s.

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Typists' fingers slow down with age, but their typing doesn't. Older typists are "more sensitive to characters farther in advance of the currently typed character than young typists," according to research published in the Journal of Experimental Psychology. In other words, they type smarter.

But at the same time -- and here's the real trip -- they aren't smarter. Researchers in British Columbia studied decision-making speeds of thousands of StarCraft 2 players and found that cognitive abilities peak at 24. Other research has found that perceptual speed drops continuously after 25. The brain is changing: the ratios of N-acetylaspartate to choline, the integrity of myelin sheathing, the connectivity of hippocampal neurons -- you know, baseball stuff.

But the runner on third doesn't notice. Indeed, this is what differentiates the relatively young and the relatively old: They decline at the same rate, but younger adults just don't notice. For a while, they just figure out how to type faster. This is the miracle of sports: The product of mental decline and physical decline isn't necessarily decline.

The missing variable is, of course, experience. A 23-year-old begins to decline at the same time that he learns how to play baseball better. The race between age and expertise is what determines whether a baseball player can have the best season of his career years after his body begins to fail.

The 26-year-old runner is a few steps off third base as the pitch is delivered, and the batter pops it 230 feet to right field -- too shallow, it seems, to get him home. To this point in the season, only one sacrifice fly has been shallower, and that one came with the right fielder running and off balance. This one is a lazy can of corn with an outfielder squared up under it. But the catch is made with a slight reach back, and Mike Trout bolts home, diving headfirst to get under a high throw.

He has lost a step, technically, but he virtually never makes an out on the bases anymore. (One this year. Twelve as a rookie, not counting five times caught stealing.) He knows what's too shallow and what's deep enough and when the right fielder has to reach juuust back to make the catch. He's the best player in baseball, and it feels like he will be forever.

"He's gonna understand it in a few years," says Adam Jones, a 32-year-old center fielder. "He's a tremendous athlete -- as am I -- and we keep ourselves in great shape. But one thing you can't stop is time."

What's behind Clayton Kershaw's decline? Maybe a loss of water content in the spongy lower back disks, for one -- or diminished fastball velocity, for another. Brian Rothmuller/Icon Sportswire

30.

There are two outs in the fifth inning in Los Angeles, and the pitcher is 30.

A year ago, he was considered, more or less unanimously, the best starting pitcher in the world, with a stretch of more than 1,300 innings -- the equivalent of six full seasons! -- with an ERA below 2.00. Now he's probably not, and he might rank as low as fifth or sixth. He allows too many home runs; his velocity has been dropping; and he keeps missing time with lower back issues. (Early byproduct of aging: loss of water content in the spongy lower back disks, leading to herniation and other problems.)

There's a lot of debate in the study of aging about what aging actually is -- when it starts, how to define it, why it happens. One thread goes like this: Through natural selection, our genes have evolved to do certain things meant to help us reach the age of sexual maturity, and they expend a lot of energy simply holding us together until then. After that point, though, selection is either much weaker or irrelevant. The genes don't give the cells instructions for how to age, and we become something more like inanimate objects, our cells degrading thoughtlessly as we come apart. Microbiologist Leonard Hayflick, a titan in the field, has argued that aging is explained by entropy -- the tendency for concentrated energy to disperse when unhindered. When the forces of our animacy quit holding us together, we just kind of break, haphazardly.

Or, as Hayflick has put it: A car has to be engineered to run. It doesn't have to be engineered to fall apart. The early part of breaking down is still mysterious, because the physical differences at that age are mostly too small to reliably detect in a lab. "We know what's happening to a 60-year-old versus a 30-year-old, but 30 to 33 -- especially somebody who is a physical freak like a major league ballplayer -- it's really tough to make detailed scientific statements," says Dr. Michael J. Joyner, an expert on human performance at the Mayo Clinic. "Just like people get better through marginal gains, and all of a sudden things click for them on the way up? People fail the same way. Marginal de-gains."

Ballplayers first notice it in the short, explosive moments. "To get to a 97 mph fastball that's up in the zone, you know you can get it there," 31-year-old veteran catcher Caleb Joseph says. "It just isn't as readily available anymore. When you're 22, it's always on. You're like, 'Do I need to get a lighter bat? Is this how it's gonna be?' "

He laughs, then pauses, deciding which kind of story he's telling. "I went down an inch this year. I'm still hitting .150."

Is it that he's not as strong? That his brain doesn't pick up the pitch as fast? It could be, but it could also be that the nervous system moves slightly slower as we age, says Corey Dawkins of Baseball Injury Consultants. Joseph could identify the pitch just as quickly, decide to swing just as confidently, swing just as powerfully as he ever did -- but the signal from brain to muscles takes a fraction of a microsecond longer to travel.

The simplest reading of sports is that we want to see the extremes: how fast a human can throw it, how far a human can hit it. But that's not quite true. If that's what we wanted to see, we'd let the pitcher get a running start, we'd let the hitters use aluminum bats, we'd let them all drink Deca-Durabolin and we'd only make them play one game a week. We want to see the extremes when limitations are put on them. We want to see what they do when we make it hard. Age is the ultimate make-it-hard. The 30-year-old pitcher throws a curveball for strike one, then he throws a fastball for strike two. It's 87.9 mph. In a start just 363 days earlier, his fastball averaged 94 mph, but today the average is 89. Less than 24 hours after this game, in fact, he will return to the disabled list, the lower back again.

But first, Clayton Kershaw throws a changeup for strike three. He throws maybe 20 changeups a year, but he sneaks one in here and it works. He strikes out the side, lowers his ERA for the season to 2.76 and leaves the game having allowed only one run. He's still very good, but baseball has become, for this pitcher, hard.

A few years ago, Justin Verlander thought his career was over. Now, he's an outlier to the aging curve -- giving hope to the rest of us. Streeter Lecka/Getty Images

35.

It's the seventh inning, the score is 4-0 and the pitcher throwing the shutout is 35.

He's been an ace for most of this decade, but in the past few years, his peers have been disappearing. Jered Weaver and Matt Cain retired last year, at 34 and 32, respectively. Tim Lincecum, 34, was in Triple-A this year until he got released. Felix Hernandez, at 32, now throws in the high 80s and carries an ERA in the mid-5s.

""We can identify with the decline. But the outliers -- they make us feel like we're immortal!"" Dr. Michael J. Joyner, Mayo Clinic

There was a point a few years ago when the man on the mound feared he might be approaching such a fate. He'd thrown an 88 mph fastball in a game, and he thought his career was ending. Now, though, at 35, he might once again be the best pitcher in the game. "Rather than stability, we have lifelong flux," wrote the authors of the StarCraft study. "Our day-to-day performance is, at every age, the result of the constant interplay between change and adaptation."

We know, or can speculate on, some things about this pitcher's body: His mitochondria -- the little factories in the cells that produce energy -- probably don't work as well as they used to. His muscles are probably losing elasticity; his tendons and ligaments are stiffer from having less water content; his bones are more prone to fractures or stress injuries. He doesn't produce as much testosterone or growth hormone as he did in his early 20s, and it's therefore harder for him to add muscle mass. "I could continue to squat the house," 36-year-old second baseman Ian Kinsler says, "but I'm not going to get any stronger anymore. The older you get, it's just about feeling good." Which sounds a lot like a description of hospice.

But we know that despite all of this, some guys get better. The numbers on the left side of the equation don't add up to the number on the right, which strongly suggests we're capable of far more than we think. There is some extra potential that exists, and if this pitcher can tap into it, why not any of us?

"We can identify with the decline because we all experience it," Joyner says. "But the outliers! They make us feel like we're immortal!"

The pitcher throws: 99.24 mph. It is Justin Verlander's 79th pitch of the day, and he records the second out of the seventh inning with it. Not long ago, Verlander had gone years without throwing a pitch so hard. He was asked this year when he thinks he'll retire, and he said he's now thinking 45. "I don't know if that's realistic," he told MLB.com, but we don't want realistic. We want to see realistic get toppled, over and over, so that we aren't burdened by the tyranny of "realistic" in our own aging lives.

By 38, even as a career is hurtling towards its end, there are still moments to celebrate. Jennifer Buchanan/USA TODAY Sports

38.

The game is in Seattle, there are 41,705 fans in attendance and the batter is 38.

He was once 23, once 26, once 30 -- in fact, he finished second in the MVP voting each of those years and won the award three times in between. But for the past seven seasons, he has played for a team whose fans have never idolized him -- have sometimes resented him as an expensive burden -- and today he is in a city that has virtually no connection to the greatness his next hit will codify.

"Complex systems -- power plants, say -- have to survive and function despite having thousands of critical, potentially fragile components," writes Atul Gawande in Being Mortal. "Engineers therefore design these machines with multiple layers of redundancy: with backup systems, and backup systems for the backup systems. The backups may not be as efficient as the first-line components, but they allow the machine to keep going even as damage accumulates."

The 38-year-old at the plate used to do everything: one of the best defensive first basemen ever, a valuable baserunner and a multidimensional hitter who mastered the strike zone and homered nearly as often as he struck out. One by one, the systems have broken down: He's a DH more often than he plays the field; it hurts to watch him run; he almost never walks; and he sets career highs in strikeouts and career lows in almost everything else. His career survives mostly on the basis of one home run per week.

Catchers' framing skills peak in the mid-20s, research has found. Batters' contact rates peak at 28 or 29. Batters swing at the fewest pitches out of the zone in their early 30s. Batters draw the most walks in their late 20s, and pitchers issue the fewest walks at about 26. Ground ball pitchers' ground ball rates peak at about the same time. The only thing that peaks with age is maturity.

Surviving in baseball is a yearslong process of learning to get through this. It takes skill to believe, in the middle of the worst periods, that this too will pass. As Kinsler puts it, the advantage of being old is that you've learned how to handle even the longest stretches of failure because "you know it's going to change. Just from past experiences."

You master the delusion just about in time for it to cease to be true.

The pitch is a little outside, but the 38-year-old reaches out and hits a soft flare into right field. It sounds like his bat breaks. The hit -- the 3,000th of Albert Pujols' career -- has been, directly and indirectly, a long time coming. The 26-year-old Mike Trout leads his teammates out of the dugout and gives the 38-year-old Pujols a long, textured hug. Then the 23-year-old Shohei Ohtani reaches into a scrum of teammates and, with a big smile, pats him on the back. The fans in Seattle give a sincere standing ovation to a man whose greatness is long gone.

There's a way of looking at the data to conclude we will all die -- 100 percent of the people who came before us did. But there's also a way of looking at the data to conclude that, in fact, I never will. I've been alive for a billion data points and I haven't died once.

To watch the 38-year-old these days is to see these two arguments smash into each other. It is to watch a dignified man walking alongside, but not yet into, the end. It's to see an athlete who was once the very best in the world fail, repeatedly, in public, and to see that it's OK -- not at all shameful -- to get worse. It's to see the smiles and the ovations among it all. It's to see that, ultimately, this isn't life and death. Just a metaphor for it.