There seems to be a developmental window for young men to get sexual experience with girlfriends that runs between the mid teens and early 20's.
If you miss this window and try to jump in later like, say, in your 30's, you are going to be at a real disadvantage because you won't know how to read women, so you might inadvertently do something that gets you into trouble, like a "date rape" accusation from a woman who thinks that you took advantage of her after she drank to much.
Also, you won't know how to recognize women who are bad until you get too far involved with them: Sluts; alcoholics in training; women who don't handle money rationally and who have debts they want some man to pay off for them; and psychologically damaged women like ones with borderline personality disorder.