When should I make an appointment for the gynecologist?

In ten minutes Go now! How young of a "girl" are we talking about? The most reasonable answer would be after her menarche, or 1st complete cycle. Unless there is some indication that there is a problem, there is no need to get worked up about seeing a gyn. Read "Taking Charge of Your Fertility" by Toni Weschler. This book is better than ANY Sex-Ed class!