It's actually not vague and wasn't meant to be insulting - the prefrontal cortex doesn't finish developing in healthy adult humans until around the age of 25 with females developing a little earlier. This area of the brain, particularly the lateral frontal pole, is responsible for long term planning and decisionmaking, it's also one of the latest developing parts of the brain that's involved in decisionmaking.

What I'm getting at is that it's ridiculous to try to argue that a 17-18 year old's abilities to plan and make decisions around the future are on par with the abilities of a 25+ year old. The average age of enlistment is 20-21 with a lower bounds of 17.

I'm not arguing that the military doesn't provide _lots_ of opportunities to people or that people only enlist for the benefits (the best I could find was a RAND Corp survey predicting likelihood of choosing enlisting of working or going to college), I'm arguing that it's shameful that we aren't able to provide similar opportunities without putting young people, who are at a disadvantage when calculating the risk involved, in shitty situations.