The Effects of Age on Hitting
by David Luciani
Published February 21, 2002

It's no secret that age has an effect on hitting but there are plenty of myths still out there.  One popular one is that a hitter's skills peak at age twenty-six or twenty-seven.  While it's true that there are certain skills that peak at those ages, some skills, such as speed, will peak earlier and other skills, such as the ability to take walks, peak much later.  Even if you don't take interest in exactly how we can study the effects of age, as many of the less statistically-inclined readers will not, you can skip to the conclusions at the end of this essay to see what insights we can gather from the actual data.

Most readers know that I take great interest in the science of forecasting and though I have written about age in the past, I have decided in this space to give some insight into exactly how we can determine the effects of age, how we can't and what the results showed.

Firstly, I am not a believer in those methods that add up the history of baseball, sorted by what "all twenty-six year olds" have done and what "all twenty seven years olds" have done and so on, making a futile comparison between the two.  That is a terrible way to analyze the effects of age because many players leave the league, through retirement or other means, long before their career is over.  If for example, you add up what all thirty-eight year olds have done, and conclude that all thirty-eight year olds hit .260 over the history of baseball and all twenty-eight year olds hit .280 over the history of baseball, you might falsely conclude that a thirty-eight year old loses 20 points off his batting average from ten years earlier.

That's entirely untrue.  Indeed, only a small percentage of twenty-eight year olds manage to remain in the majors until they are thirty-eight because of their inability to overcome the effects of age.  In fact, most twenty-eight year olds would evolve into much inferior hitters than the thirty-eight year old population we can study.  We don't have the terrible results they would have accumulated had they stuck around because they are long since retired.

One way that we can study the effects of age is somewhat simple but requires a great degree of database manipulation.  Fortunately, I can save you the work because we've already done it.  To study the effects of age, we need to compare players who played at both ages we are interested in comparing.  To accomplish this, first we took the entire history of baseball and neutralized each year's statistics for the effects of the era and, where data was available, the effects of the park.

For example, we start with all hitters who played when they were both 18 and 19 years of age.  If they didn't play at both ages, then obviously we can't use them to consider the difference in their performance.  Secondly, we neutralize the plate appearances so that we scale down the year in which the player appeared more to the amount of time in the year in which he played less.  For example, if a player had 30 hits in 100 plate appearances when he was 18 years old and he had 50 hits in 200 plate appearances when he was 19 years old, we scale down the 50 hits to the number it pro-rates to over 100 plate appearances.  Once we have these totals, we can add up all the reduced results and plate appearances over both years to find out the total effect of age on categories.  I've summarized and simplified an entire chapter's worth of analysis into one paragraph here so if you don't follow the method, we will return to this in future essays.  I am more interested in presenting some of the results at this point.

We took the entire history of baseball, with adjustments for era, league and park (where possible) and analyzed the effects of age on every category of performance.  When the analysis was finished, and our first version of this was completed in 1994, the curves were so extraordinarily smooth that it seemed clear that we had stumbled onto some hidden equations of nature.  Since 1994, we have continued to add the new data into the model though it has not significantly changed any equation.  We do keep updating it, just in case something new should appear.  We use these equations, in combination with many other factors, when making the forecasts you see online.

Let's start by looking at the graph for hits per plate appearance (not to be confused with the more popular hits per at bats):

Now, I have not altered the graph in any way and these are the real results we have from our post-2001 analysis.  Any result above 1 means that a player's ability to get hits, per plate appearance, improves relative to the year before.  Any result below that means that he declines.  As you can see, a player's skill up until about the age of 25 sees his total number of hits per plate appearance improving relative to the year earlier.  As you can see, he improves more between the ages of 19 and 20 than he does between the ages of 24 and 25 but in both cases, he is still improving.  By the time he is 40, he would be facing an annual decline of about 5-10% per year on hits per plate appearance compared to the year before, which is why few last that long because losing 5-10% of your hits over the span of a few years will quickly destroy your ability..

Remember of course that this is the average hitter.  A small percentage of hitters will dramatically improve/decline beyond expectations and a small percentage will actually do the opposite.  The point here is to study the typical expected effects of age on a hitter.  The reason I observed above how a hitter does per plate appearance rather than by at bat is because at bats per plate appearance are also affected by age, actually declining until the player is about 29 or 30, mostly because he gets better and better at taking walks as he gets older and walks are a rare skill where a player continues his improvement into his mid thirties, largely because taking walks is not as much a physical skill as it is one of experience.

So having said that, let's study an untypical 18-year-old who hits .290 over 500 major league at bats.  It's rare for a player so young to do something like that but let's take a look at this hypothetical situation.  The at bats should decrease slightly as he gets older, the hits increase as above, and this is the expectation for the player.  Notice just how perfect the line comes out, without any smoothing.  This is using the chart results from our raw study on the effects of age on batting average and the line is fantastically smooth, so smooth that we can be fairly certain that our analysis has not yielded coincidental results:

As you can see, our .290 18-year-old has done something extraordinary and according to even the average expectations, he should quickly develop into a .310-.315 hitter over the next several years.  He should stay above .300 until around thirty and then see a steady decline.  In the unlikely event he were to still be playing into his late thirties, by the age of thirty-eight he becomes just a .235 hitter.  By his early forties, our one-time batting title contender, probably long retired by now, should be expected to be just a .188 hitter.

I present this graph not to reveal the entire study on the effects of age, though I will talk much more about this analysis over the season and will focus on other categories such as power and speed.  The point here is to demonstrate why a .290 18-year-old should be expected to become better than a .300 hitting 21-year-old.  Let's take a look at a .300 hitting 21-year-old, using our same age effects information:

Our .300 hitting 21-year-old actually should not get much better average-wise, though he still has some expected improvement.  This hitter, by the time he is twenty-four years old, should be peaking around .305-.310.  Beyond that, his decline below .300 will come at the young age of just twenty-seven and if he is somehow still playing when he's into his forties, he should be expected to become a .183 hitter by the age of 42.  In fact, if our .290 18-year-old and our .300 21-year-old are playing at the same time, our .290 18-year-old will move ahead of our .300 hitter about two or three years from now.

Now the problem with any analysis such as this is that we're considering averages and as with any set of averages, there is a wide deviation in actual, eventual performance.  Some great players in modern times have exceeded the expectations that age data would otherwise imply (consider Roberto Alomar) and others have fallen short (Tony Batista for example).

One result from our analysis is that we are able to say, with some degree of certainty, when skills actually peak, on average.  Please note that these ages do not mean that a player should achieve a career-high in these categories.  It means that the player's skill per plate appearance in that category should actually peak at this age if they were to still be playing, but the best base-stealers will achieve career-highs much later than their peak speed because they will play more when they are older. 

Whether you agree or disagree with the analysis, these are rooted in fact and I present the results only for your entertainment.  None of these conclusions are rooted in what I or other people believe or say and they are all simply the result of calculations.  Accept them then for what they are, the results of a numerical analysis of every hitter in baseball history:

• A hitter's batting average ability typically should peak at twenty-four.

• A hitter's ability to to hit doubles typically should peak at twenty-four.

• A hitter's ability to hit triples typically should peak at twenty-one (meaning most hitters experience their triples peak while still in the minors and thus achieve their eventual major league career high in triples much later than the peak of the same skill).

• A hitter's home run skill should typically peak at twenty-six (which may back up the popular "26 with experience" theory).

• A hitter's ability to take walks typically should peak at thirty-five (really!).

• A hitter's ability to avoid striking out typically should peak at twenty-nine.

• A hitter's ability to steal bases typically should peak at twenty-three.