A Typical Career Arc for a Batter

It has long been conventional wisdom in baseball that batters hit their peak in their late 20s and go into decline after the age of 30. But inasmuch as this conventional wisdom pre-dates the days of massive computerized databases and spreadsheets, can it be true, or is it the baseball equivalent of an old wives tale?

To answer that question, I went to the Play Index feature of Baseball-Reference.com. I was able to find (thanks to a paid subscription to Play Index) the career OPS+ statistic* for all inactive or retired players from 1901 through 2009 who had at least 3,000 plate appearances. As of yesterday, there were 1,416 such players. I selected 57 of them by the simple (and essentially random) method of picking #1 Babe Ruth (career OPS+ 207) and every 25th batter thereafter: #26 Jeff Bagwell (149), #51 Babe Herman (140, and so on, down to #1401 Bill Kellefer (63). The resulting sample represents great-to-poor batters, players in all eras from 1901 onward, and careers long, short (but not too short), and in between.

I indexed each player’s single-season OPS+ statistics to that player’s career-high OPS+. For example, Babe Ruth’s career looks like this:

Season Age OPS+ Indexed
1914 19 50 0.196
1915 20 189 0.741
1916 21 121 0.475
1917 22 162 0.635
1918 23 194 0.761
1919 24 219 0.859
1920 25 255 1.000
1921 26 239 0.937
1922 27 181 0.710
1923 28 239 0.937
1924 29 220 0.863
1925 30 137 0.537
1926 31 227 0.890
1927 32 226 0.886
1928 33 208 0.816
1929 34 193 0.757
1930 35 211 0.827
1931 36 218 0.855
1932 37 201 0.788
1933 38 176 0.690
1934 39 161 0.631
1935 40 118 0.463

After making the same computation for the other 56 batters, I plotted the indexed values for all batters against age, with the following result:


The ages at which the players peaked (index value of 1.00) range from 22 to 36, but the polynomial curve tells the tale: major-league batters tend to improve gradually as they approach ages 29-30, and then to decline gradually thereafter.

That is a broad generalization, based on a random sample of batters. Even within that sample, and certainly across the entire population of batters, there are many-many exceptions. Babe Ruth is a prominent exception. He peaked at age 25, by the measure of OPS+, declined somewhat, improved somewhat, and did not go into his final decline until the age of 37.

In any event, the conventional wisdom stands up to scrutiny, but (like conventional wisdom about many things) it is a rule of thumb, not an iron-clad law of behavior.
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* Definition: “OPS+ is OPS [on-base plus slugging percentage] adjusted for the park and the league in which the player played,” where the league average for a given year is 100. Thus “An OPS+ of 150 or more is excellent and 125 very good, while an OPS+ of 75 or below is poor.”