This post begins with two connected theories:

a.) Defense ages better at certain positions than others, due to the specific physical demands of each position.

b.) Paying 37-year-old Adrian Beltre $13M in 2016 wouldn't be as bad as you might think.

Obsessive A's fans (and Angels fans?) are wondering what Adrian Beltre's game will look like at the end of a five or six-year contract. Let's say the A's up the ante tomorrow and offer Beltre a six year, $78M deal - $13M per season. The scariest part of this deal is Beltre's age 36-37 seasons in 2015-2016. Clearly, he will be in his decline phase...but what if he's still an above-average defender at that point?

Is it more likely for a 37-year-old corner infielder to retain their defense skills than an OF, 2b-SS, or C?

The discussion continues after the jump.

Several baseball researchers have done work on fielding aging curves before. Here's an article penned by Tom Tango on shortstops, in which he found that shortstop defense peaks somewhere between the ages 24 and 28, and shows a steady drop from there. This makes sense intuitively - at a position that requires great agility and technique like SS, the peak age is going to one where a guy is young enough to retain all his physical gifts, but still old enough to have received years of daily professional practice at the position.

I was hoping to find a similar article on third basemen, but I couldn't any more aging curves articles written by Tango either for his own blog or in the Hardball Times archive. My guess is he started including those fielding aging curves articles in the Hardball Times Annual, which I don't own. Or it may have become proprietary info once he started working for MLB teams. Either way, that one article is the extent of Tango's work on aging curves on the Internet, as far as I know.

Then I found the goldmine. This article by Jeff Zimmerman, "Determining a Player's True UZR," provided exactly the info I was looking for. It's filled with great research and charts, but here's a few key results, (quoting Zimmerman's article, in the blockquotes):

The preceding data seems to indicate that there are 3 categories for rUZR: Outfielders – All three groups show about the same decrease of ~0.125 rUZR per year of age. Corner Infielders – These both are pretty steady over the years with values close to 0, but 1B seems to decline a little more than 3B and its decline rate is closest of any position to the overall decline rate. Middle Infielders – These two positions were a little confusing in that they had a trend of an increase in regressed UZR over the years. I needed to look into them a little more. I selected the players that played 4 straight seasons at SS and looked to see how much of change there was from year to year and the results were the same. The players seem to having an improving rUZR to a peak around age 27 and then decline after that.

Once you look at the positional rUZR, the overall rUZR values makes a little more sense when the following 3 trends are combined: General overall decline(outfielders)

Increase at the beginning (middle infielders)

General overall leveling (corner infielders)

So, all else being equal - salaries, talent, scouting reports, etc. - I would rather sign a first baseman or a third baseman to deal into their late-30s than a middle infielder or outfielder. This correlates to my hypothesis from the beginning - the positions that require greater lateral movement are going to suffer the most decline. But a guy in his mid-to-late 30s isn't going to suddenly lose his ability to cleanly field ground balls that are hit to him.

But how about that last sentence - can we test that? Well, I wish had some SQL skills, or a great database of historical advanced defensive statistics. But I decided to look at what I could find for some historically great third basemen on baseball-reference, to see how they aged.

"Greatest defensive third baseman of all-time" is obviously a subjective statement. But I got some great ideas for players to research from the comments section of this article by Dave Cameron, written after Beltre signed his one-year deal with Boston last offseason.

In that article, Cameron discusses Beltre's defensive chops, and discusses where Beltre belongs in the pantheon of great third basemen. He mentions that Beltre finished as the sixth best defender in all of baseball in the Fan's Scouting Report after the 2009 season. He also shares this quote that I hadn't read before:

“[Beltre is] clearly the best [third baseman] I’ve ever seen in person,” said Tampa Bay Rays manager Joe Maddon. “I think [Evan Longoria] is good, I used to think Scott Brosius was really good. … [Eric] Chavez was good, but Beltre was stupid good. I think Beltre is the best who I’ve ever seen with my two eyes – defender, not just third baseman, but defense.”

Wow, high praise. Beltre's defense reputation seems to be that of one of the best at his position of all time. Now, people might one day put Zimmerman and Longoria in that camp, too. But they aren't useful for this discussion, simply because they haven't aged yet. So I looked at the stats for three other guys, including one of Beltre's age peers: Scott Rolen.

Note: If you click on each player's name below, it will take you to his BRef page. The numbers that follow after his name will be defensive range figures. The figure "RF/9" is "Range Factor per 9 innings" - 9 x (the amount of putouts + assists the player made) / 9 innings. The figure "lgRF/9" is the league average of the same statistic. I will always list the player's personal RF/9 figure first, and the league average figure second. What we are looking for is to see if some of the historically great 3b defenders remained above-average at their position in their late-30s. Good defense means that the first number is higher than the second.

Brooks Robinson - age 36: 3.24, 3.00; age 37: 3.47, 3.12; age 38: 3.10, 3.16; age 39: 3.02, 2.94. "The Vacuum Cleaner" was a well-above-average defender at 3b at age 36 and 37. Even at age 38 and 39, he hovered around league average. That age 37 defensive season was the 2nd-best performance of his career, behind only his age-30 season in 1967, when he posted a 3.52 RF/9 against the 3.13 league average.

Graig Nettles - age 36: 2.97, 2.95; age 37: 3.07, 3.07; age 38: 2.98, 3.07; age 39: 2.80, 2.79; age 40: 2.93, 2.88; age 41: 2.73, 2.72. Remarkable. The man was a league-average defender through age 41.

Scott Rolen - age 35 (2010): 2.87, 2.54. Rolen had his first healthy season in a while, and he was outstanding defensively. As our only contemporary comp in this analysis, we have UZR data for him, too. Here's his fangraphs page. As a 35-year-old, he still posted one of the better defensive scores in the league last year. And just ahead of him was Adrian Beltre.

In researching for this article, I feel like I've learned that a.) Adrian Beltre is among the best 3b defenders of all time, and b.) that the group of historically great 3b defenders retains their defensive skills into their late 30s.

Now let's talk money.

The free agent market has seen an explosion this offseason, and once the smoke clears, it's expected that the FA value of a win will rise to $5M+. Previous calculations of the dollar value per FA win in year's past have been made at $4M per win or $4.5M per win. This makes sense - the FA market is experiencing inflation, and the baseball economy has remained robust, despite a downturn in the world economy. We should continue to expect FA inflation, thanks to a couple of factors: a.) MLB Advanced Media revenues continue to put more money in team's coffers; b.) the Marlins' new stadium opening in 2012 should improve their revenue and increase their budget; c.) the world economy will eventually improve; d.) the Rangers and other teams' new cable deals will produce new revenue streams. In short, the game is very healthy, and will continue to grow.

What happens when we take that $5M per win free agent value in the 2010-2011 offseason, and assess inflation of 7.5% on it for six years?

At that rate, the FA value of a win - market rate for acquiring wins - would be a staggering $7.1781466M by the year 2016 - the final year of a six year, $78M contract for Adrian Beltre ($13M per year).

Acquiring a two-win player, at market value, in that market would cost $14.35M. Even if inflation is only 5% per year for the next six years, the FA value of a win would be more than $6.5M. In either scenario, Adrian Beltre essentially earns his salary in 2016, even at age 37, if he is a 1.5-2 win player. And we have already established that a historically great defensive 3b typically retains his defensive skills into his late 30s. So I think it's feasible to project that Beltre might still provide a win with his glove in the last two years of that hypothetical six year deal.

Adrian Beltre could be a slightly-above replacement level hitter at age 37 in 2016, in the final year of a six year, $78M deal, and still potentially earn his money in the final year of the deal.

After researching this piece, I would feel confident offering him a six-year, $78M deal. And I think it's crazy to think that a 5 year, $80M deal is a better offer for the team (as many have argued in the previous threads). It's a free extra year, and as I've showed here, he may actually have value in 2016.