> Disqualified from signing up, but you can age surely?

To an extent. The US military caps military service at 30 years (20 for reserve officers), last I checked. So if you enlisted at 35 (right at the cutoff for the usual maximum age to enlist), you'd be forced to retire at 65, whereas if you enlisted at 17, you'd be forced to retire at 47. 65 (IIRC) is right on the edge of when age starts becoming a significant risk factor; for the vast majority of military personnel, it ain't gonna be a factor.

There are of course exceptions to this based on rank or specialization; for example, had my grandpa been promoted to General, he would've been able to serve longer, but he wasn't selected for the promotion so he aged out as a Colonel (or at least that's how he explained it to me).

Majority on the ship are going to be under 40. Most people start their military careers between 18 and 22, and most people leave at it before 20 years of service.

Squadron commanders, senior medical officer, Dept heads and the CO and Admiral would most likely be in their forties or early fifties.

On average, people who start out older tend to end up older, after a given period of time. That's my theory, at least.