Would you flirt with a girl who is under age knowing that later u could ask her out when she's 18?(6 yr age gap)?

With a 6 yr age gap, puts you in your twenties. In my twenties I would never flirt with a teen never mind think about asking one out. By the time the maturity level comes close she would need to be near 25, which put you in you at 31. To me it seems like a waste of time and the chance of meeting someone who might be better suited for you.

That would be a child compared to you .. So it's really not a great idea.  Anyway, who's to say she would be attracted to someone that much older. 

That's Grossness  :/

At 18, it is quite possible that she will not remember your flirting with her at age 12 as being a pleasant thing to remember.

Better to wait.