I'd let my son start dating seriously at 14.
My daughter can start at 21. (I'm only kidding, I'd let her start dating at the same age, 14).
Way I see it, you tell them they can't date and they'll date all the wrong people just to get under your skin. You let them date with responsibility and educate them on the hazards (moral, emotional, and physical) and they'll be responsible at a younger age. It's like with alcohol. Europe lets children as young as 10 drink a tiny bit and then teach them responsibility with drinking as they grow older, which is why they have such a small drinking problem in comparison to the U.S. In the U.S. it is so strictly banned until 21 that by the time they hit high school they are eager to get drinking and only binge drink and never take alcohol for what it is, an enjoyable beverage with pleasant physical side effects. They just go strait from sober to smashed and miss the whole point of the drink.