The armed forces as a whole are much younger than people expect. I was an officer at 20.

Part of the answer is that officers haven't always been selected on merit, but were selected by class. So they didn't need prior service or training before being appointed to the rank.

Part of the answer is that there were no academies and so there were proto-officer ranks where officers learnt their trade. Think of navy midshipmen or army ensigns. For naval midshipmen in the wooden ship navy it was a great advantage to be young: it gave more years of service to reach post-capain where fame and fortunes were made.

You also need to be careful to be sure you're not being misled by fraud. Promotion was often by seniority. So having someone 'on the books' when young could advance their career when they joined at a later age.