Trying minors as adults in a U.S. court of law - Yay or nay

Prompted by a not heated but 'pointed' debate with a friend...

I, personally, am against it, because minors obviously aren't adults. Their brains don't function the same way, and in the eyes of the law, they aren't allowed to handle their own affairs (unless they're emancipated), can't drink a beer, can't vote, and can't go off to war. It's also never uniformly applied in trial and sentencing.

People often fight me on this by saying "some people can't be rehabilitated", but I challenge that notion as we don't often try it, and most people say this because they want the 'criminal' to face the punishment they think is deserved. I would even wager most people don't even favor rehabilitation (though I have no survey statistics to prove this, just a guess). Since we don't have a rehabilitative prison system, I don't see how we can make the claim...and nobody is 'born' evil anyway.

Contrary thoughts? I'm interested in seeing what people with legal backgrounds or criminal justice backgrounds have to think, but all opinion is welcome.

Aren't American prisons too full already?