It doesn't. I'm not sure where you get the notion that football players are required to attend college to play in the NFL. They just need to be 3 years removed from their high school graduation.
That's why I said above that a kid could go hone his skills in semi-pro ball or train privately in the meantime. He doesn't have to go to college. He just has to wait three years.
EDIT: By the way, there's also an age limit in baseball. Players have to be 16 to sign a pro contract, if I'm not mistaken. Why didn't you mention this when you brought up Albert Pujols? Because 15 is generally considered to be too young to play in the MLB, just like 17 or 18 is generally considered to be too young to play in the NFL. See? Age limits exist everywhere.