Recently, I've been asked several times about age limits to become a pilot in the US Air Force. Age is actually not considered a 'medical' standard. In the USAF, flight surgeons use a hefty document, titled the 'Medical Standards Directory' (MSD) to determine if pilots and other special duty personnel in the USAF are medically qualified for a particular job. But age is nowhere to be found in this document. The age limits for military pilots is actually an administrative standard.
USAF
In order to apply to the board for undergraduate flight training (UFT), applicants must 'not exceed' their 30th birthday by the start date of the first available UFT class. Because the board hires for training quotas approximately 18 months in the future, typically it is recommended to be 28 years old or younger at time of application. If an interested applicant can not meet the age criteria, an age waiver up to age of 35 years old is possible, otherwise an exception to policy is required.
US NAVY & MARINE CORPS
To become an Aviation Officer Candidate (AOC) in the Navy or Marine Corps, applicants must be at least 19 years old and not yet passed their 27th birthday upon commissioning. Waiver requests for those up to 29 years old are considered on a case-by-case basis.
US ARMY
For both regular commissioned officers and warrant officers, applicants to flight school must not have passed their 33rd birthday at the time of enlistment. The 'High School to Flight School' program is unique among all services for military aviators. This program allows high school graduates to apply to become a warrant officer and can enter this track directly without a college degree or prior commissioning.
US COAST GUARD
Coast Guard pilots must either already be USCG officers or previously graduated from another armed service's flight school. The Direct Commission Aviator (DCA) program allows military pilots from other branches to join the Coast Guard directly as aviators. Applicants to the DCA must be at least 21 years old and not reach their 35th birthday by 30 September of the fiscal year in which the selection panel convenes.