A player's baseball age indicates how old he is as of June 30 in a given season.
The purpose of baseball age is to establish one age per season for each player, even if a player has a birthday during the season and spends parts of the campaign at two different ages. A given season will often be referred to as a player's "age-x season," with "x" being the player's baseball age.
Note: Baseball age is not an official metric, but it is widely used in baseball literature and media.
Mike Trout was tied for seventh all time in homers through a player's age-25 season with 201 through the conclusion of 2017. Although he was 26 years old by the end of the 2017 campaign, it is considered his age-25 season because he didn't turn 26 until Aug. 7, and, thus, was 25 on June 30, 2017. As a result, Trout can be said to have tallied 201 homers through his age-25 season.