Democracy in America questions the wisdom of allowing teenagers to join the military. Money quote:
Of course, raising the minimum age for joining the military is an extremely impractical proposal. It would probably require a significant raise in pay to recruit enough soldiers if 18-year-olds could no longer join. Adults tend to find recruitment campaigns based on macho athletic fantasies and evocations of the World of Warcraft experience somewhat less convincing than teenagers do. But for a military that expects to spend much of its time drinking tea and building collaborative relationships with local community leaders, rather than blowing things up, that's probably a plus.