Below The Belt
Any person under 18, if they cant buy alcohol, tobacco or participate in sexual activities legally, it doesn't matter if its a he or she, if they are under aged they are considered below the belt.
Example 1:

Guy 1: hey dude, look at that hot chick over there.

Guy 2: yo chill dude, shes below the belt.

Guy 1: damn!....... she still looks good though.

Example 2:

Woman 1: OMG, look at that guy over there coming out the comic store, he is a hottie.

Woman:2 umm, i think you need to put your glasses on.

Woman 1: why?

Woman 2: hes clearly below the belt, and besides hes one of my students.