>>In his view, you could not become a teacher until you had completed a (rather lengthy) tour of combat or served 10 years.
I know. I’ve read the book several times. But, we don’t have a never-ending war against the bugs and our enlistments aren’t as long as they were in ST. So, for practical purposes in the USA, I propose the minimum age restriction.
At the beginning of the book’s chronology, they don’t have a war against the Bugs either. And today there are 8 year enlistment terms available.