There are only two things that determine whether you're old enough to do something -- whether you understand what the hell you're getting yourself into -- and whether you're willing to accept responsibility for it if it blows up in your face.How many years you've been alive is ultimately meaningless -- except in as much as it gives parents a general sort of idea as to whether their child is likely to understand what they're getting themselves into. Small children, for instance, can't really comprehend shades of grey -- where a decision or choice can have different answers depending on the circumstances. For them, everything is black and white.