text: It's a question of whether the person has the ability to understand what they are consenting to.  Society has decided generally to protect people that otherwise cannot protect themselves.  Young children and people who are mentally incapacitated for example.  Each state has drawn a line in the sand to say that as a matter of law, a person under age X is too young to understand what they are consenting to.  It is an obviously imperfect system, since people mature at different rates, but for manageability sake the states have drawn a line.