I can absolutely believe this. My brother was in the Marines. He wanted to become admiral. His performance was good. According to him, if he got promoted to captain before 40, he'd make admiral, otherwise he was on a track to become a military attache in a random country. He was on track to become captain at 41, but every year someone who had been an enlisted man first got promoted to captain a year early.
When he wasn't promoted early, he quit with a full pension.
Ummm, the Marines don't have admirals, and you should make captain in the Marines pretty fast, since it's only O-3, as opposed to O-6 in the Navy. You sure he wasn't in the Navy?
When I think about it, I think he was an enlisted man in the Marines for Vietnam, then an officer in the Navy. He left the military service somewhere in the 80s.
This is a much older brother that I am not close to.
And note that Navy (or Coast Guard) Captain is understandability a much higher rank than in the other services, O-6, Colonel elsewhere, the last rank before flag AKA General or Admiral. I can see an unofficial requirement for reaching O-6 before 40 for then being in the running for flag rank.
When he wasn't promoted early, he quit with a full pension.