Over the years, numerous violent acts have been perpetrated by minors. The standard age for adulthood is 18, but adult understanding often comes earlier. For this reason, one questions wisdom of trying someone under the age of 18 as an adult. We just assume that upon reaching a certain age, one has developed a complete understanding of what's right, what's wrong, and what the consequences are. I think that by age fifteen, one is fully aware of both his actions and the consequences they beget, and so should be responsible for them.
Some teenagers are simply too young for full understanding.
For instance, earlier this year, a young girl in Michigan was shot in school by one of her classmates. The unbelievable thing was that they were both in the first grade. Now it is understood that in the first grade, one doesn't have nearly the level of comprehension that a 15 or 30 year old has. Despite this however, there were school officials in Michigan who wanted to try this 7 year old in such a way that by the time he is 18, he will have already begun adult punishment for his offense. This first grader may have had some understanding of what he'd done, but the idea of prosecuting him as an adult is outrageous.