Good question. I googled it. It would appear that you must already be 35 when you first declare you will be running. Looks like you have to be 35 just to even be a candidate
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.[1]
In March 2009, Representative Bill Posey, a newly elected Republican from Florida's 15th congressional district, introduced a bill, H.R. 1503, in the U.S. House of Representatives. Had this bill been enacted into law, it would have amended the Federal Election Campaign Act of 1971 to require candidates for the Presidency "to include with the [campaign] committee's statement of organization a copy of the candidate's birth certificate" plus supporting documentation
https://en.wikipedia.org/wiki/United_States_presidential_eligibility_legislation