It's getting harder and harder these days to determine whether or not a girl is old enough to be checking out. Girls from ages 15-21 can all look the same these days. If a girl is wearing a backpack(like a hello kitty one), chances are she's too young and you should look away before being called out for being a creeper. If she has no backpack than it's safe to look, but still be cautious if you choose to engage in conversation.
Benji- "That girl is cute but she looks like she might be young."
Joe- "Gotta use the backpack rule."
Benji -"What's the backpack rule?"
Joe- "If she's wearing a backpack she's too young."