J.F.K. was 35 when he won the 1962 election over Nixon. A good reference for the various positions reads:
Who Can Be President?
Someone who is
1) a natural-born citizen of the U.S.
2) at least 35 years of age
3) a resident of the U.S. for at least 14 years.
http://www.infoplease.com/ipa/A0878573.html