The NCAA does not impose an age limit on playing college football. There is, however, a four-year limit on the number of years a student can play football.
The length of eligibility rule is meant to prevent colleges from enticing their star players to stay in school for longer than they should. If a player starts playing football in his sophomore year, he could still have a year of eligibility left after graduation. Such a player can continue on to graduate school and play for one more year. Another rule governing college football is that the player cannot have previously played professional football.