Well I am a 24 year old male, without children so here is my $0.02 I think that it depends on your daughter's ability to stand up against peer preasure. Worse things can happen in a closet at a party than on a date in a movie theatre at ANY age. (hopefully you are familiar with those closet, bottle spinning, dare games) Bottom line, is your daughter mature enough to tell a guy to buzz off? Does she have a life focus for herself past high school with the understanding that sex can potentially damage or delay those goals. I have been under the assumption you are concerned only about your daughter having sex, but I think the child's maturity level and sense of self really determine at what age a child is ready to make their own decisions.
Now, My mother allowed myself and my brother to have our girlfriends over in our room at the age of 15. Her reasoning was that with the proper education, and knowledge of consequences, teenagers DO in fact make responsible decisions. My brother nor I, got a girl pregnant and have grown up to be successful adults. (although I have a hard time with spelling) <GRIN>
Whatever you will do will be insignifigant, but it is very important that you do it. ~Mahatma Gandhi