I was never fond of being in the military. It's just not my style. However, I think the military does a pretty good job of recognizing and rewarding merit. This is probably more true today than when I faced the prospect of being drafted.
Some 21-year-olds probably shouldn't be allowed near a fork-lift and some 19-year-olds would probably do fine. You only have to be 25 to be a member of Congress. If I thought being older would improve the situation I would support that.
A friend of mine has a son who is a "retired" Marine. He now works as a warehouse manager. Next time I see him I will ask. It wouldn't surprise me if he started driving a fork lift at 17 when he enlisted.
My oldest daughter was in the Coast Guard for almost 10 years, and the second graduated from USMC boot camp at Thanksgiving last year. She learned to drive a Humvee in Australia earlier this year, so I’m sure she could handle a forklift ... but 21 is a magic number for lots of employers. It used to be 18.