Matt Steinglass doesn't want to send 18-year-olds to war:
[T]eenagers should not be firing automatic weapons in America's name. We should raise the minimum age for soldiers on combat duty to 21. Behavioural research since time immemorial shows that teenagers are prone to doing ridiculously ill-advised things, particularly under the influence of peer pressure; mortality actually goes up 200% during adolescence even when we're not shipping the rascals off to Afghanistan; and MRI mapping shows that the brain doesn't finish developing white matter until you're in your 20s.
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.