This is a rather controversial issue, and just for the hell of it, i'm gonna repeat what everyone else has said...
Wait until you feel ready.
I think that trust is the biggest thing involved. If you can't trust the person, you can't enjoy the sex to it's full potential (atleast not in my opinion). Having sex is a lot more of a personal thing than many people believe. Even though lots and lots of people are just jumping into the sack with anyone they're attracted to these days, that doesn't mean that it's what everyone should be doing.
I personally could never let anyone eve try sex with me unless i trusted them. Unless i knew for sure that they were not going to hurt, ditch, or make fun of me behind my back, it wouldn't be happening.
I am however only talking about full out sexual intercourse. Other forms of foreplay, in my opinion, are a whole other deal. Not that oral sex can not have consequences, but i feel that it is less serious.
Anyways, the point is, as everyone else said... there is no "golden rule" telling you when to have sex. For some people it's just whenever they feel horny, for others it's whenever they have been in a loving, honest relationship with someone they truly trust.
Which one you choose is up to you.
Oh, and it's not just guys that get impatient about sex! A good friend of mine used to live by the "golden rule" that if the guy didn't screw her the first time she wanted to (usually within the first day of meeting the guy), then he must be gay, and she'd move on.
I wouldn't however suggest following in her footsteps...