You look about 15

An exaggerated way of saying someone in their late teens/early twenties looks young for their age. They may be chance look actually 15 but usually they look a little older than 15 but younger than their actual age.

22 year old: Could I get a litre of vodka please?
Cashier: Do you have ID on you?
Friend: HAHAHA! YOU LOOK ABOUT 15!