Going back to the 1950s, officers began their training at 13.
US military prep programs were integrated into scouting and local high schools with plenty of local adult oversight.
I think 17 with parents consent was the minimum age for the military proper.
We also had (and still have) Cadet Programs, which operate more like something like the Scouts - one afternoon or evening of parade a week, and camps, throughout the year. Some run in schools, and some in the community in general.
But that was separate from the actual practice of taking in boys as Cadet Midshipmen, or Junior Recruits, to train full time on Naval bases, for eventual deployment as sailors to the fleet.