how old do you start college in america

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The NAIA does not have an age restriction; however they do take away seasons of competition for any participation in sports at a comparable level of competition after September 1st of your high school graduation year.
There is an age limit for NCAA Division I and II sports. The NCAA allows a one year grace period after high school graduation for DI and II schools. One year after your high school class graduates is when your eligibility will start to be affected in all sports except for hockey, skiing and tennis. The eligibility clock does not start for hockey players and skiers until after their 21st birthday. Tennis players start losing eligibility 6 months after they graduate high school.

References:

http://www.athleticscholarships.net/question/am-i-too-old-to-play-college-sports
http://www.goodschoolsguide.co.uk/choosing-a-school/state-schools/secondary-school-admissions