Three Year Rule
It is not socially acceptable to go out with anyone within the three year age bracket. This only applies if the younger of the two is 14-16 years old.
"Hey dude, i'm dating this chick who's 15"
"Wtf dude you're like 18, ever heard of the three year rule"
"Wtf dude you're like 18, ever heard of the three year rule"