More on Changing Hitter Aging Curves

A few days ago, I looked at the possibility of major league hitters no longer showing any hitting improvement, on average, once they debut in the majors. I believe both the banning of PEDs and teams being able to evaluate MLB ready talent are the keys to this change.

To start off, I am going to look at a few of the question raised from the first article. The first question I would like to answer is “Was the aging curve change a one year fluke?”

To begin with, here is the wRC+ (link) curve going back six, ten year intervals.

Generally hitters have aged the same over the years with the exception of the past two 10-year intervals. From 1993 to 2002, players reached their peak a little slower and took a little longer to decline compared to the previous intervals. The other and more important change involves the most recent 10-year interval. Batters start at their peak and then decline at a faster rate. I expected the drop to be more than the 1993 to 2002 interval, but it was more than any other time frame. I wonder if pre-1993 amphetamine usage helped hitters perform better than previously thought.

The second question I would like to answer, “Is the aging rate change a one year occurrence or has it been happening for a few seasons?”

In the original graph from the first article, I looked at the 2006 to 2013 and 1998 to 2005 time frames. There was quite a difference in wRC+ between the two. I went back and got the values for each 7-year interval between and including the original curves.

The hitter aging peak became less and less defined the closer the time range gets to the present. The aging curve has slowly been changing over the past seven seasons.

The third and fourth questions I will answer are, “Are there less young players in the majors right now?” and “As the number of young hitters have decline are the ones playing at their offensive peak so they won’t improve?”

I went back and looked at the number of harmonically matched plate appearances for each of the 10 previous intervals I examine. Here is the percentage of matched pair plate appearances for six age ranges.

The number of matched plate appearances has dropped since the 1963 to 1972 interval until the last 10-year cycle. The graph is a little hard to read the exact values, so here is the percentages of matched player seasons from age 25 and younger (called young hitters from now on).

Time Frame	25-years-old or younger
1952 to 1962	17.7%
1963 to 1972	23.2%
1973 to 1982	20.1%
1983 to 1992	16.7%
1993 to 2002	13.1%
2003 to 2012	14.9%

The change makes sense. During the 70’s, players were granted free agency and teams were more likely to keep their young talent in the minors until they were major league ready. The percentage of young players continued to decline until the last interval when it increased a bit. So for 40 years, the number of young hitters declined by over 10% points (23.2% to 13.1%), but the aging curves stayed the same. From 1993 to 2002, the least amount of young hitters played, but the aging was the steepest compared to the other seasons.

Finally, I wanted to answer, “Do non-MLB hitters have their peak at 26-years-old?” I looked at the wRC+ aging curve for Triple-A batters from 2006 to present (extent of FanGraphs minor league database) and here is the aging curve.

Players in triple-A still have an up and down curve with a peak at age 25.

In conclusion, I think two items are causing the change in the curves. The first in the ban and harsh penalties on PEDs between the 2005 and 2006 seasons. Besides the ban on muscle building drugs, the crack down on amphetamines may have caused players to no longer chemically peak. The second effect is teams being able to better know when a player is able to contribute at their peak MLB level. I think this ability to evaluate talent may have existed before the PED ban, but increase production from PED masked the change. In the near future, I will look to see which stats are causing the change. In the meantime, let me know if you have any more questions.