Is There an Age Requirement?
There is not technically an age requirement to enter the NFL. The NFL never mentions a specific age in its rules, and bases eligibility on other factors.
The Real Requirement is Time
The restriction to enter says that a player must be at least three years removed from the end of high school, or that he must have completed at least three years of college.
Are Three College Seasons Required?
For many players, this results in a mandate to play three college football seasons, but this isn't always the case. Some players take time off before entering college or play at private academies or different leagues.
Additionally, the act of "red shirting", or sitting a college player for a full year before he plays, means that players are then often eligible after two seasons of playing.
Reasons for the Restrictions
The main reason behind the NFL's restrictions is that the NFL is a very intense and tough sport. It requires that a person's body be strong and developed. Even a supremely athletic kid coming out of high school will not have the fully mature strength, bones and body of an older individual.