I look young, so when I tease a girl they will often come-back with something like
Girl: 'You look so young, how old are you?'
M: "How old do you think I am?"
Girl: 'Bla bla bla'
M: "Okay, now add 3. (pause) Now subtract 2. Multiply that by 1.5. Now give me the square root." By this time they're a little taken aback, so I touch them lightly on the elbow or shoulder and say "I'm just fucking with you. I'm 22. And you?"
This one works better on the smarter girls. Dumb/older ones I might just say "Probably younger than you grandma. You look like you just got your hip replaced "

If you got a better one I'd love to hear it, I hear the age comment in like 1/3 sets.