35 years old
a person has to be at least 35 years old
Barack Obama is 49 years old (birthdate: August 4, 1961).
The minimum age is 35.
In order to be eligible to be the President, you must be 35 years old. Note the distinction between elected and eligible. Being elected does not technically fall into play.
To become President, you have to be at least 35 years old, have lived in the US for at least 14 years, and be a natural-born citizen.