Age 27 Cohort:
This group started at 173 players with 54 only playing one season leaving 119 for my purposes, and they averaged 5 full seasons each. Out of the 119, 49 (41%) maxed out their wRC+ in their first full season and 44 (37%) maxed WAR. Both of the groups that maxed out in year one averaged 3.2 full seasons in the big leagues.
The same thing we have seen since the age 25 cohort continues, a clearly declining performance trend in aggregate from the time they show up until they leave. In year 1, these players are hitting on average at nearly 90% of their max, so there is almost no chance of a large increase in subsequent seasons.
Age 28 Cohort:
Sample sizes are going to start becoming a big issue again as only 110 started and 38 only played one 300+ PA season. The remaining 72 averaged only 3.7 full seasons. For those that were maxing wRC+ or WAR in year one, both groups included 32 of the 72 (44%) and averaged 2.7 seasons and 3 full seasons respectively.
The chart does show an increase in WAR from year 1 to 2 do to an anomaly, but the hitting shows the 90% of peak on average and decreasing from there. You can ignore the spikes in age 40 and 41 seasons as there was only one player accounted for there, Davey Lopes, who happened to hit pretty well those two seasons. Without him it drops off like all of the others and ends at age 38. You can see that by WAR the entirety of their decline is pretty much done by 30 years old, only their third seasons and thus the short careers.
Age 29 Cohort:
This group is nearing the point where it might be worth ignoring anything you see with a starting group of 62 that gets whittled down to 41 players with more than one full season. Those 41 averaged 4.6 full seasons in their careers, longer than the 28-year-olds because of a few guys that hung around awhile and the small sample. One was Hideki Matsui who was a professional long before 29, but not in the United States. I will discuss two others in a moment. Out of our 41 players here 23 (56%) had their max wRC+ in their first full season, and almost 50%, 20 out of the 41 had their best WAR. At a coin flip for whether we have seen their best or not immediately we have definitely hit the point where any real growth as a player is unlikely or purely luck driven. Those two groups of year one max wRC+ and WAR had average career lengths of 3.7 and 3.1 years respectively.
Like the last group we see a little uptick at the end, and these were two of the odd players from this group that hung around. Actually, Raul Ibanez is still hanging around currently in Minnesota with the Royals, and the other is a former Royal too in Matt Stairs. Again, in reality this group is pretty much all declining from year 1 on and almost all are finished by their late 30s. There is a large spike in WAR for ages 32 and 33 and a smaller corresponding one in wRC+ because 4 players had their best season at 32 and 5 players at 33 which is a significant amount out of a pool of 41 players. Those two years along with the first full season of the cohort comprise over 70% of the players and so it is probably just a sample size issue that we see the early 30s uptick here.
I am done with the cohorts, or at least running through them all the first time. Players that play their first full season at 30 or older were mostly ignored. There were 92 of them total and about 80% of them maxed in year one or only had one full season, so to chart a growth pattern would be ludicrous for the other 18 to 20 players who didn’t all come up at the same age. Next I will summarize this all and try and point out several other things that I learned from breaking these cohorts apart so that you can get the full picture, or at least as much of the picture as I have managed to see.