I feel that she probably meant it as a joke...is this girl your friend?
Then no matter what her tone is, it was a joke.
But if she is not a friend, not even remotely, then it depends on how she said it.
It may still be a joke, or maybe she really asked that.
I don't think there's anything too rude abt it. Maybe her definition of magic is still the stereotypical one, that's why she just wondered out aloud something.
Has she seen your magic?
Remember, when we show magic to someone and NOT the stuff they expect (like a simple "pick a card" or a trick which screams "MATHS")... then we become their only connection to a world the know nothing about. Because many see magic tricks, but few get to have a magician friend or talk to a magician in an informal setting. So she probably just asked you something hoping you would help her understand this whole "Magic" thing better...maybe she thought that close-up magicians like this are MEANT to grow out of magic...maybe she had a little sibling who grew out of it.
So all in all, if she's a stranger, I guess she was just being a slight bit ignorant, but nothing I would feel offended about