text: why is it illegal? why is it illegal for an 18 year old to be with a 17 year old, but not for an 18 year old to be with an 80 year old? 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.