A line needs to be drawn somewhere (since I doubt that you are you suggesting that a five year-old child should be considered "adult" enough to consent to sexual activity or that a ten year-old should be able to legally drive.) Where we draw the line is based upon culture, tradition, and sometimes it will shift up or down based on societal or technological changes. (And _all_ societies are like this, not just this one; a line is drawn in the sand to divide the adults from the children.)

Yeah, I know it sounds lame, but as a matter of fact you _will_ have a better perspective upon the folly of your youth once you are well past it.