At the moment, I have a baby daughter, not a teenager, and my response probably should not be trusted because my first two thoughts are "never" and "over my dead body," and my slightly more open-minded response is "when she's 18 and I get to ride shotgun, as in I sit in the shotgun seat with a real shotgun pointed at the head of my daughter's would-be boyfriend."
I suspect, based on my totally unreasonable responses, that this is a question better left to those who have actual teenagers.
Still, here is my somewhat more thought out but still totally theoretical response: 16 or 17 years old. I can't help thinking that 14 and a freshman in high school is just too young and impressionable to date. But I kind of also like the idea of my daughter having dated a bit before going off to college so that she isn't completely naive at a time and in a place that I can't really do anything about it. But I guess I will have to find out the hard way in 15 to 16 years if I am lucky and sooner if I am not.
On last thought: I was a pretty obedient kid (oldest child and a child of immigrants) and didn't think at the time that I had time for dating. And I was shy with girls I liked. But on top of all of that, I am pretty sure that the flat-top that I wore until the very end of my senior year in high school was a pretty effective birth-control device and all-around girl-repellent. Which now gives me an idea for my daughter when she grows up a bit...