A long time ago I remember reading a statistical analysis on the aging of baseball players. One sign of a player that would age well was the player's inability to strike out. The theory was relatively simple. If a player strikes out a lot when they are younger, they will only do worse as they age and their bat speed gets worse.
The paper (which I'm struggling to find) used George Brett as an example. He struck out a paltry 7.8% of his at bats over his career. As a modern day comparison, from 2011-2013 when Miguel Cabrera won three batting titles, Cabrera struck out 13.7% of the time. George Brett was able to last in the big leagues until he was 40. Although not the elite hitter he once was he was productive and even won a batting title in 1990 at the age of 37.
Other players in this same mold can be found. Hank Aaron and Tony Gwynn both come to mind.
Now, strikeout rates have been increasing in baseball, especially in the last two decades, so "good strikeout rate" is now somewhat relative. David Ortiz's strikeout rate of 13.7% of his plate appearances in 2016 may seem high, but it's relatively low compared to many of his other power hitting peers. As an example, Mike Trout struck out 20.1% of his at bats in 2016, and that was a decline from prior years.
However, one of the anomalies in baseball in the 2000s was Albert Pujols. Despite strikeout rates going up, Pujols continued to strike out at very low rates. From 2001-2011 in St. Louis, Pujols struck out only 9.5% of plate appearances. A paltry number for a power hitter in this century. His worst year was his rookie year when he struck out 13.7% of the time.
It's one of the reasons some people felt a long term contract for Albert Pujols may not be that bad compared to others that received big long term contracts. The belief was that Albert Pujols would age better than many of his peers.
Albert Pujols' strikeout rate as an Angel has stayed somewhat consistent. It's been 11.2% over his 5 years as an Angel. However, as many of us know, Albert Pujols is not the same today. In 2016, his WAR of 1.4 was a career low, even below his injury plagued season of 2013. So is Albert Pujols not aging well?
It's at this time I realized, that "aging well" is a relative term. Very few players can play baseball into their 30s. So "aging well" may in fact simply be a statement that a player is capable of just holding a job in baseball into their late 30s. Not that they can hold a job at an All Star level of play. George Brett had a WAR of 1.7 in his age 36 season before a fluke-ish 4.1 WAR in his age 37 season. He never had a WAR above 1 again.
While there are the occasional Hank Aarons or David Ortizes of the world that can produce All Star performance into their late 30s, it's very rare. A player simply being able to hold a starting position into their late 30s is by itself testament to a player aging well.
I'm reminded of this fact because today Ryan Howard, one of Albert Pujols' first basemen peers was bought out of his 2017 option year by the Phillies. With negative WAR values in 2014-2016, it's likely that Howard's career is over. If he hadn't signed his contract extension so early, he likely would have been out of baseball at the age of 34 or 35.
Earlier this year, another of Pujols' peers Prince Fielder had to retire. While his retirement was due to injury, his numbers were already beginning to look bad with a 1.9 WAR in 2013 during his age 29 season and 1.5 WAR in 2015 in his age 31 season.