First time your anal flower gets penetrated by a penis or an object large enough to resemble it (e.g. a strap-on dildo in case of a lesbian couple).
It's also called loosing your anal virginity. Or as Sarah Silverman calls it: loosing your real virginity. Indeed, oral or vaginal sex happen for some of us even before we reach our sexual maturity. Sometimes not because we are ready or feel like it, but just because everybody does it. But the day you are ready for anal sex, only then you can say your mind is open enough from a sexual POV. Only then you can honestly say you're not a virgin anymore when it comes to sex.
My husband still thinks I was a virgin when we met, because he deflowered my pussy. What he doesn't know, is that I had my anal defloration 5 years before that - and enjoyed lots of sex since. ;)
Oral & anal sex are indeed 2 great ways of enjoying your youth, and still saving your "virginity". And that's good not necesarily because some men still appreciate that, but mainly because it'll save you from unwanted pregnancies.