What age do you have to be to run for President?

You must be 35 years old, which is stated in the Constitution. You also must be born in the United States, and have lived in the U.S. for the last 14 years.

From the U.S. Constitution Article II, section 1:
"No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President; neither shall any person be eligible to that office who shall not have attained to the age of thirty five years, and been fourteen Years a resident within the United States"