Never give a straight answer to anything you don't want to reveal (I've heard some guys argue never give a straight answer to a woman ever, which is over the top IMO)

...for you age fits in this category. Make her question a joke; ask her how she knew you looked old for your age. When she asks what you mean, say you're only 15, but she can't tell the bartender or they won't let you in anymore.

If you're not comfortable completely hiding your age from her, later tell her in a short story when she's much more attracted to you and you're building commonalities. Age difference won't matter as much then. Also tell it more as a story rather than a straight answer... makes it more engaging and meaningful.