I'd go with never to lol. But realistically - when I was growing up, I was told no dating until 18 and based on my experience I learned this: once teens decide they want to date, they will - not matter what you say. If you forbid it, they just go behind your back. I had wonderful Christian parents, and I knew right from wrong, but I meet a guy and had to hid it so instead of going out openly, we snuck around. And when you can't go out in public, there are limited activities you can engage in.
Yes, I became a teen mom. Yes, my own fault. But, now that we are older and wiser my husband and I have decided that we would rather allow our girls to date under our watchful eye then have them go behind our backs. Will this keep them from having sex? No - but banning dating won't either. We've told our girls they can only go on group dates until highschool. In highschool they will be allowed to date provided they follow curfew and are honest about where they are and with who. If they break curfew or get caught lying, dating priviledges will suffer.