Prime Time: When Does A Player's Career Reach Its Peak?

When does a baseball player's prime begin and end? That question, which has been hotly debated for a long time, recently prompted columnist Joe Posnanski to put down his pen in favor of a bar graph that contradicted the growing conventional wisdom that the prime years of a player's career extend into his early-30s.

Although Posnanski's chart was illustrative of his point, the underlying premise had a few flaws. In particular, Posnanski used cumulative data over 111 seasons to contradict recent sentiment. By failing to segment the data, there was no basis from which to identify the alleged shift in players' aging patterns.

"Great" Offensive Seasons by Age, Since 1901

Note: Great seasons defined as those with a bWAR of six or better.
Source: Baseball-reference.com

Posnanski's analysis wasn't comprehensive (something he freely admitted). Rather, it was a simple look at the most common ages for "great seasons", which were defined as having a bWAR of at least six. As evident in the chart above, which is a recreation based on Posnanski's criteria, exactly one-half of the 1,084 applicable seasons were turned in by a player between the ages of 25 and 29. The chart also displays sharp declines in the number of great seasons produced by players as they progress throughout their 30s. However, to what extent do the same trends exist today?

In order to answer that question, the 1,084 great seasons were allocated across seven age ranges, and then each segment was compared to the total. Not only does this approach allow for the examination of more recent trends, but it also accounts for the growing player pool since the start of the expansion era, providing better context for an era-by-era comparison.

Percentage of "Great" Offensive Seasons by Age Group, Since 1901

Note: Great seasons defined as those with a bWAR of six or better; Bold border indicates highest percentage in an age group.
Source: Baseball-reference.com

The chart above indicates a gradual shift of great seasons toward the 30-something end of the spectrum. After peaking at 41.6% during the war years, when many of the game's younger players were overseas, the percentage of bWAR>=6 campaigns by 30-year olds has plateaued at 35% over the last 20 years (see last graph below). Looking more granularly, the past 11 years have seen over 7% of great seasons turned in by players age-35 and up, a level not to far from the high watermark set during World War II. Although some will undoubtedly attribute the recent exploits of older players to the recent performance enhancing drug epidemic, WAR is a comparative statistics, so presumably, the advantage should cancel out (unless older players either disproportionately took or benefitted from PEDs).

"Great" Pitching Seasons by Age, Since 1901

Note: Great seasons defined as those with a bWAR of six or better.
Source: Baseball-reference.com

Like their offensive counterparts, pitchers have historically had their best seasons between the ages of 25 and 29. However, pitchers have exhibited a little more staying power. Since 1901, 33% of great offensive seasons were compiled by players over 30, but for pitchers, that rate is nearly 39%. What’s more, there have been several periods when pitchers over the age of 30 have outperformed their juniors in terms of great seasons. During the live ball era of the 1920s, these older pitchers turned in over 60% of the bWAR>=6 seasons, a rate that steadily declined before reversing course in the 1970s and peaking again at over 50% in the 1990s. Over the past decade, the rate has returned to a more modest rate of 33%, which is line with most of the other eras in between the aforementioned outliers.

Percentage of "Great" Pitching Seasons by Age Group, Since 1901

Note: Great seasons defined as those with a bWAR of six or better; Bold border indicates highest percentage in an age group.
Source: Baseball-reference.com

Further down the age-scale at age-35 or older, the rate of pitchers having great seasons has also fluctuated more wildly than for hitters, with two peak periods over 15% in the 1920s and 1970s to go along with two valleys below 5% in the 1960s and 1980s. It's hard to get a handle on why peak performance by pitchers has been far less uniform than for hitters, but it could suggest that pitching is a less physical skill (i.e., older pitchers can compensate for lost velocity with location and pitch selection) as well as one more prone to random variables like injury (i.e., eras in which top pitchers stay healthy will exhibit more great seasons by those in the older age group).

Great Seasons by Age-30+ and Age-35+ Pitchers and Offensive Players, Since 1901

Note: Great seasons defined as those with a bWAR of six or better.
Source: Baseball-reference.com

A lot of effort has been spent trying to define prime, but ultimately, the real answer has to be applied on a case-by-case basis. Although most players will conform to a general trend, the ones we really care about are those who transcend it (which explains the prominent names on the list below). In his conclusion, Posnanski asks "what are the chances that Albert Pujols will still be a great player at ages 37, 38, 39, 40"? But, can we really answer that question by looking at the aggregate performances of a group that is so vastly inferior? Players like Pujols are great because they aren't like everyone else, so, like a prime number, their only apt comparison is themselves.

Best Seasons by Older Pitchers (38+) and Hitters (37+), Since 1901

Note: Sorted first by age and then by bWAR.
Source: Baseball-reference.com