How old should you have to be before you can volunteer for military service?

This question about sexuality (which is marked NSFW BTW) garnered a lot of responses indicating that people feel those much younger than you should be off limits for intimacy as they are not fully grown and capable of making decisions.

It makes me wonder now if we are allowing young people to volunteer to defend their nations inappropriately. Should we prevent those under 22 from joining the military?