That's a new deflecting response I'd like to try out when some nubile honey bunny asks me how old I am, but I never get the chance anymore. Soon after the recession sank in, it became gauche to dress up, and I started wearing a t-shirt and jeans to dance clubs instead of a jacket and tie. It must've been that type of clothing that made younger people curious about my age because literally no one has asked me since I switched. Just last week someone asked, only half-certain, "...you're over 21...right?" when pointing out a bar area that they thought I'd like.
Even though I haven't gotten the chance to try it out, it still sounds like it would work for anyone. If you're youthful looking, it emphasizes this good quality of yours and has a nice mischievous ring to it. If you're noticeably older and mature-looking, it shows how big your balls are that you're accusing her of wanting you and only being worried about whether you were old enough. And the self-deprecating humor in it shows how secure you are about your increasing age. In either case, a deadpan delivery with solid eye-contact (and maybe a barely visible smirk) is the only way to go. If you say it with the slightest hint of interest or eagerness, it'll sound totally creepy -- definitely do not smile.
Clearly this line should only be used when you sense an interest, not if she asks in disgust like uggh i mean how old ARE you anyway?