Went out with a chick and she asked me how come I've only got braces now, when I'm so old (I'm 21).

I was stumped- at first I was debating answering seriously, that I'd been prevented from doing so by parents, and then I considered a firm 'so', realizing it might have been a shit test. But I didn't think that that was smart enough and might even come off as rude so I didn't say anything.

Tl;dr couldn't figure out a clever response in time so I failed.

How would you have answered it?