Truth be told, if you're under 18 you're legally considered to be an adult.
True story look up just about any news story about someone under 18 being charged "as an adult." "As an adult" is a a simile (like or as) According to http://www.dictionary.com/ this is what a simile is:
"A common figure of speech that explicitly compares two things usually considered different"
An adult is considered different from a child (or legally, a minor) and that cannot be debated, at least on a legal standpoint... well almost... on some levels.
Since the "minor" is being charged "as an adult" this LEGALLY means they are an adult. (As also meaning they ARE, "as you were" = "do what you are doing"). So if I were under 18 and was accused of certain crimes (any involving bodily injury or attempted bodily injury). I knew what I was doing because I am an adult!! (Minors, legally don't know they're doing, but are still held responsible and are still sent to jail and what not... boy, the contradictions keep mounting on top of one another).
Adults are ALL considered responsible (introduce me to one, PLEASE) and therefore they're RESPONSIBLE for their actions and as some retarded hicks love to say "If you can't do the time, don't do the crime" (**Throws rocks at them to shut them up and make them realize how much of an asshole they are***). Minors are considered to be partially responsible for their actions but their actions are so heavily regulated that they're asking to get caught. The pressure is simply too high for most if not all to break, if only in the US.
Long story short: You're under 18? You're a minor. You're under 18 and commit a crime? WELCOME TO THE RESPONSIBLE WORLD OF ADULTHOOD... 20 TO LIFE!