Fourteen seems too young to me. I think sixteen at the least...Definitely not fifteen, and ideally seventeen or older. People who want to have sex at the age of fourteen (though this doesn't mean not lusting for sex.) probably aren't ready for sex in the first place.
Sex means different things to different people, and in fact, most people don't even know what sex means to themselves at the age of fourteen, actually I'm not sure that a lot of people ever realize what it means to them.