Player Aging Patterns Over Time

Baseball fans often talk about when a player is in his prime or peak years. In this article I examine whether or not that peak has changed over time. Also, I look at how older players have done over time-are they a growing percentage of all players? Are older players doing better than older players used to do?

To compile this data, I used the Lee Sinins Complete Baseball Encyclopedia.The table below shows what percentage of players who had 400 or more plate appearances (PAs) in a season were a certain age. The age which had the highest percentage is in red.

It might be hard to tell if there is any pattern there. But in recent decades, older players have been playing more. The graph below shows this.

It sure looks like players over 35 and 39 are playing more. I am not sure why or what it means. There is another interesting age/timeline chart at the end of this article.

I then looked at all players in 4 25-year periods who had at least 10 seasons with 400+ PAs and found at which age did they most often have there best year and what was the average age at when they had their best year. The stat I used was RCAA or runs created above average. It comes from the Lee Sinins Complete Baseball Encyclopedia. Here is how he defines it: "It's the difference between a player's RC total and the total for an average player who used the same amount of his team's outs. A negative RCAA indicates a below average player in this category." It is also park adjusted. Runs created was initially a Bill James creation which is an estimate of how many runs a player creates for his team based on his stats.

So the table below shows what was the most common peak age and the average. There was a tie in the third period.

It does not look like much change over time. Then I found the top 100 seasons in RCAA for each of the 10 decades of the 20th century. The table below shows the average age for the top 10, top 25 and top 100 in RCAA in each decade.

Things do fluctuate a bit here. The highest average age for the top 100 is the last decade. The next table shows how many players 36 or older made in to the top 10, 25 or top 100 seasons in RCAA for the different decades.

Ted Williams at age 38 had the third highest RCAA of the 1950s in 1957 with 113. Hank Aaron had the 9th highest in the 1970s at age 37 with 73. The next table does the same thing for players 40 or older.

It is rare for a player 40+ to make this list. Ted Williams in 1960 had an RCAA of 53 (tied for 49th in the decade). He was 41. Willie Mays was 40 in 1971 when he had an RCAA of 47, good enough for a tie for 55th. Now Barry Bonds had the highest RCAA ever in 2001 (169) at age 36. Most fans know he did very well in 2002-04 as well.
I have an article on Bonds where I discuss his aging patterns.

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