The last hot stove myth that I previously wrote about has to do with player aging.
Players peak at 27 and old players are worthless — Players peak at 29 — 30. And just because a guy is past his peak doesn’t mean he’s not valuable. The aging process is gradual, more like the Minneapolis Metrodome than an Egyptian pyramid. If a guy was good last year, even if he’s in his mid-30s, he’ll probably be good next year. Now, the older he gets the more dangerous long-run contracts get, but one- and two-year deals are fine.
This may not seem like much of a myth, as the conventional wisdom has long been that baseball players peak around 30. This, it turns out, is a myth of sabermetrics. Many methods have been used, aggregating performance into age-buckets, identifying the most-common (or mode) age at which players have their best season, and calculating average changes in performance from age to age. These methodologies suffer from various problems that could induce bias; therefore, I set out to conducted my own study in away that might satisfy my concerns.
I gathered a sample of major-league baseball players over 86 seasons who had significant career lengths to track performance over time. Without a career, there is no trajectory to follow. I tracked their performances over ages 24 — 35, throwing out younger and older years when only the best players typically play. Following individual players over time allowed me to set baselines for each player according to his ability, while controlling for changes in playing environments which may fluctuate quite a bit over the course of a player’s career. For example, a hitter who started his career in the mid-1980s and finished in the late-1990s might have appeared to have continuously improved, when in fact his higher offensive statistics reflected a jump in league offense. I used z-scores to measure player performance in terms of standard deviations from the league average to measure performance relative to one another.
I poked, prodded, and tortured the data, and it screamed that peak performance occurs around age 29 since the early-1920s, possibly 30 for more-recent players, not matter what changes I made. Some aspects of performance peak earlier while others peak later, but overall players tended to gradually improve until 29 and slowly declined after that. I think it’s particularly interesting that the average pitcher in the sample continues to reduce his walks until his 32nd birthday, while strikeouts peak at 24. Batters also improve in their ability to draw walks into their early-thirties, even after their hitting and power have declined. It appears that players continue improve mentally even after their physical skills are eroding. Maybe there is something to veteran know-how.
Age is often used as a reason to chastise GMs for picking up players past their prime. Though old players may not be what they once were, the evidence indicates they can still be valuable. According to my estimates, a hitter who has a .900 OPS at his peak would be expected to post around an .850 OPS at 35; a pitcher with a peak 3.5 ERA is expected to post around a 3.75 ERA at 35. Yes, age saps athletic skill, but the stock of skill being diminished is also important.
I should also note that previously, Phil Birnbaum argued that the quadratic shape of my estimated aging function could bias the peak. Certainly, this could happen. I looked into the possibility using polynomials of higher magnitudes and fractional polynomial methods that do not require symmetry. The results still hold, so I went with the more parsimonious functional form.