The oldest person ever to play MLB was Satchel Paige, who, at the age of 59, made a major league appearance twelve years after his Major League career had ended.
The oldest active player is San Francisco Giants pitcher Justin Verlander, who is currently 41 years old.
Note: Paige, O'Leary, O'Rourke, Jennings, Street, McGuire and Evers each made one token major league appearance years (or even decades) after their careers had otherwise ended.
The careers of Altrock, Miñoso, Austin, and Latham had also effectively ended many years before their final major league appearances, though each were brought back for token appearances several times (spanning several seasons) before their final game.
Julio Franco is the oldest position player to appear regularly.