A TRUE GENTLEMAN NEVER ASKS: Rob Neyer has an informative piece on the history of baseball players shaving years off their ages. Neyer shows that almost 25% of players in the major leagues in 1952 had what used to be called "baseball ages."
In his review of the 1952 players, Neyer has the following observation regarding Latin American and black players of the time, who were the groups most likely to reduce their ages by more than one year :

Historically, most players who've invented birthdays simply shaved one year. But most of the Spanish speakers and the American blacks figured if they were going to fib, they might as well get their money's worth.

And of course, the Yankees' "El Duque" Hernandez continues that tradition, shaving four years of his age in a fiction that receives lip service from the Yankees' front office and from no one else.
With the phenomenon being this widespread, we may have to adjust our expectations of players' career paths. Analysts have long pinpointed age 27 as being the peak time period for hitters (isolating an expected peak is much more difficult and inexact for pitchers, for reasons which I won't get into here). Is it possible that much of the data we've relied upon in reaching that conclusion has been false?
One other aside. According to Robert Creamer's biography Babe: The Legend Comes to Life, Babe Ruth did the reverse: he added a year onto his age in his youth due to some confusion (I don't have the book with me and don't remember the details), and according to creamer, never bothered to correct it even after the mistake was demonstrated to him during his career. Babe Ruth was always an original.