For the Navy see below, STA-21 now encompasses OCS which I'm assuming you're inquiring about
https://www.netc.navy.mil/programs.html
Age: STA-21 Pilot Option candidates must be at least 19 years old and able to complete degree requirements and be commissioned prior to age 27. Maximum age limit may be adjusted upward to age 29 with active duty or prior active duty service computed on a month-for-month basis and approved by PERS-435A. No waivers beyond 29.
NROTC - Must not have reached 27th birthday by June 30 of year in which graduation and commissioning are anticipated
• Applicants with prior military service may be eligible for age adjustments for amount of time equal to their prior service, on month-by-month basis, for maximum of 36 months, if they will not reach 30th birthday by June 30 of year graduation and commissioning are anticipated