David Goffin is only 21 years old. It is hard to get past his age. Before his fourth-round match against Roger Federer, he mentioned that he used to have posters of his idol plastered on the walls of his childhood room. Who believed the posters weren’t still up, that he wasn’t still sleeping in that room? With his unaffected face, wide-set eyes, floppy blond hair, bony shoulders, and narrow chest, he makes a baby face like Ryan Harrison look geriatric. Goffin seemed just happy to be there. He was what’s known as a “lucky loser.” Ranked 109th in the world, he didn’t qualify for the main draw, sneaking in only when a player withdrew at the last minute, yet he had somehow made it into the second week at Roland Garros. Goffin hung with Federer — in fact, played better than Federer through the first set, winning it 7-5. Goffin struck the ball early, fearlessly, and threw himself into his swing with such force that his torso sometimes twisted in the air, his hair flying. He wasn’t afraid of the net, coming in 35 times and winning 21 of those points. Goffin played like he was having fun, like a kid. After one show of shots — dashing right, scrambling left, knifing the backhand volley — he raised his arm to salute the crowd and bask in their adulation.

After the first set, Goffin regressed, and Federer did what Federer does, at least for the time being. He set up his moves well in advance, disguising his drop shot, waiting for the moment to be aggressive, and then seizing that chance. Afterward, Federer praised Goffin’s performance, said that the Belgian reminded him of himself at his first grand slam, gave him a big hug, and — literally — patted him on the head.

Much of the fuss about Goffin’s youth was really fuss about his youthful looks, not his actual age. (I half-suspect that his fanboy act was calculated to demolish Federer’s expectations. If so, it worked!) The players in the 19-21 range generally are considered prospects, judged by their potential and talent more than by their success. No one blames Milos Raonic or Bernard Tomic for failing to threaten in a major. With four players so rooted in the top spots, and a few supremely talented ones below them, how could they? What’s more surprising is that no one expects them to threaten because they’re too young. That wasn’t always the case.

David Goffin is already 21 years old. When Roger Federer was 21 years old, he won Wimbledon.

Sloane Stephens is only 19 years old. It is so tempting to call her the future of American tennis. Granted, that hope has a lot to do with her off-court persona: her charming way of dealing with the press, her compulsive tweeting, her demeanor, her dimple when she smiles, which is all the time. She speaks just as naturally and cleverly about kicking her orange Fanta habit as she does about the burden of expectations placed upon her. She does seem young, in the way that people who make you feel good about the future often seem young. None of this would have mattered if she couldn’t back it up with her play, but her play at the French Open was impressive and occasionally inspired. Stephens has a big serve — she can crack it in the 110s — and powerful strokes, heavy shots that can do damage in the slower conditions on clay. For an American, she even seemed comfortable on the surface, sometimes sliding like a Spaniard.

Against the sixth-seeded Samantha Stosur in the fourth round, Stephens held it close. She even had a chance to serve for the first set before she tightened up. After two double faults, a Stosur winner, and a total shank, she was broken at love. Stephens’s game needs work. Though her footwork is sometimes precise and efficient, she often sashays in a lackadaisical way. Against Stosur, she had a habit of settling into long crosscourt rallies that pushed her farther and farther out of the court, pulling her own shots toward the center. Stosur, for her part, easily ran around her backhand to hit inside-out forehands, one of her favorite shots. After Stosur broke Stephens to start the second set, Stephens shuffled around the court, slumping and throwing up her hands. She had plenty of chances to get back into the match, even after going down 5-1, but when she had the chance to serve to level the set, she played like she knew she would lose. So she did, quickly.

I watched her play and felt patient. She has the tools, the body, the pedigree. Her mother, Sybil Smith, was an All-American swimmer; her father, John, who died in a car accident in 2009, was an NFL running back. At least in the interview room, she clearly has the confidence and poise. From her mother and her aunt, who flew to Europe to be with her when she was feeling overwhelmed, she seems to have excellent support, and her coaches say she responds to instruction and is improving all the time. What she needs, it seems, is experience. Age will give her experience; experience will give her mental toughness and a better feel for the court.

Or so you hope. It doesn’t always work out that way. Take Stosur, 28 years old and the reigning U.S. Open champion. She certainly drew on her wealth of experience to beat her young opponent yesterday, but even then, she was clearly rattled when she had trouble closing out the match. Throughout her career, she has struggled under pressure. Success didn’t help. No one would call Stosur a model of mental composure.

Wherever Stephens went, she was asked what it was like to still be in the tournament while the Williams sisters were out, with the none-too-subtle suggestion that she was the heir apparent. (Of American tennis? Of being a media darling? Of being black? It wasn’t clear.) But there will never be another Serena Williams.

Sloane Stephens is already 19 years old. When Serena Williams was 19, she had already won a grand slam, and when she was 20, she would win three more.

Tennis used to be a kid’s game. Pete Sampras won his first major when he was 19. Boris Becker won as a teenager. You don’t even have to look that far back. Maria Sharapova, who will replace Azarenka as no. 1 if she makes this French Open final, won her first major at 17. The young were expected to come up and destroy the old. This was the natural order of things.

I don’t completely understand why things are different now. In this year’s French Open, 37 of the men were thirty-something years old. As Sports Illustrated‘s Jon Wertheim points out, that’s the highest proportion of men over 30 at a major during the Open era. None of the younger players were expected to make a deep run.

There are lots of theories. You hear that the game requires more strength and fitness than it used to, which favors more mature bodies. Most 20-year-old bodies, though, have finished maturing — and have greater natural strength and endurance than older bodies. Better training methods and nutrition science have allowed athletes in many sports to prolong their careers beyond what was recently considered possible, but the older ones start from a disadvantage. They’re growing weaker every day. (Of course, it’s true that parts of the brain do take longer to mature.)

You hear about the importance of experience. Point well taken. But didn’t experience matter 15 years ago? Martina Hingis, for one, didn’t need decades of professional experience to develop a superior court sense and confidence. On the women’s tour, it seems like the more experience the players have, the more likely they are to self-destruct. No lead is safe. It doesn’t seem to matter who is competing: Every match is a roller coaster, every tournament wide-open. Six unseeded players made the final 16 at the French. The no. 1 player in the world, Victoria Azarenka, smashed her racket like a petulant child as she lost to Dominika Cibulkova in the fourth round. Recently, the no. 2, Maria Sharapova, has been the most consistent player, but she usually either bulldozes her opponent or claws back from a deficit by will alone, instead of doing what you’d expect the superior, more experienced player to do: adjust her tactics and play with patience.

Maybe I’m overthinking this. When you do something for longer, you tend to get better. The older players don’t have to worry about their technique. Rafa Nadal does not have to head to the practice court to work on his forehand. The older players know their bodies — know when to drink and when to eat, when to push it and when to take a break. They’re not afraid to rest. They don’t drink orange Fanta. They move more efficiently; they know just how to exploit their fancy strings, to make the ball move in ways their aging arms alone might not allow. They can buy the time it takes to develop their mechanics and their strategies. No one is telling them they should stop; fewer and fewer people are telling them they’re too old. Tennis players are now more professional than phenomenal. There may never be another Jennifer Capriati, teenage superstar — but hopefully, there will never be another Jennifer Capriati, troubled burnout. One reason the women’s players are coming up more slowly is the implementation of an age restriction known as the Capriati Rule.

At the same time, the older players can see the end, even the ones who can still beat any kid on any given day. Federer has looked worryingly human this tournament. The consensus favorite on the women’s side, Serena, lost in the first round. You had to wonder — and many did — whether the rare discomposure that Williams showed during her loss to Virginie Razzano wasn’t due to her growing awareness that, at 30, her time is running out. She wanted this tournament, maybe too much.

Perhaps a young player will come along and do something that the other players aren’t doing, with the aid of new technologies or not. They’ll change the game, and in doing so change the limits of our expectations. That will be a gift to the sport. Until that happens, we have a different kind of gift: the chance for older players to accelerate through the rankings like the kids used to do, for Brian Baker to come back from devastating surgeries at 27, or Varvara Lepchenko to come from nowhere into the second week of the French Open at 26. When Lepchenko was Stephens’s age, she was just trying to get by, sleeping in a guest room offered by a kind tennis enthusiast in Allentown. Age matters, and it doesn’t.