(Also don't ever mention age to a woman.)
So instead of
"You should be an actress. I really can't tell how old you are haha. You could pass for anywhere from 21-35"
You could say something a bit more engaging like
"Hmm are you an actress or something? You seem to be playing the good girl, but deep inside I feel like you might be super naughty.. 😎"
It's not a perfect line by any means. (If there was one we'd all be using it.)
But something like that is more engaging and more likely to get a flirty response that you can work with.