I don't think it's a big deal in general—healthy curiosity about your body and all that!—it's more, considering the kids that I've helped raise, and the questions they would have asked/how they would have broadcast what they learned to their peers, I don't know that I would have been comfortable discussing it with them candidly. Stating that it's something that happens "when you're growing up" and ending the discussion there would be fine; going into a detailed explanation of what all is going to happen and when to expect changes (including menstruation, breast development, etc)...I dunno that I'd feel completely comfortable introducing those topics at that young of an age, if that makes sense.