But isn't birth - or first menstruation, or what have you - a much cleaner line? Shouldn't we be going by those, instead? No? Of course we shouldn't; sometimes the obvious choices for where to draw a line are so off from what would be reasonable that we're forced to be arbitrary.

Isn't 20 years a much cleaner line than 17, for age of consent and drinking age? Why not go with those? You have to pick something.