16, if I had to pick a number. But of course, it's not cut and dry. Some kids mature at different ages. And what exactly defines "dating"? For the purposes of this discussion, dating is defined as a 1 on 1 activity that both parties consider a date.
For most parents the main concern is the level of physical intimacy that might occur on that date. There is a clear double standard among males and females, like it or not. When it comes to dating, fathers are generally much more protective of their daughters than all other scenarios. And safety, both physical and emotional safety is a main concern. If the person is even a slight threat to safety, then all bets are off.
However, the more a parent doesn't like the person, the more attractive they become. Doing something your parents don't like is a powerful way to prove your own independence...and girls like the "bad boys". It's not just a saying.
"So how do I prevent my kid from dating if I say no?"
First, you have to have a good reason for not wanting the date. Feeling like your kid is starting to not need you is not a good reason. If you feel that the person has low aspirations, or is antisocial, then those are good reasons.
Focus on self-respect. Children should respect themselves enough to not even want to associate with people with lower aspirations, self -destructive tendencies, and antisocial behaviors. This is a purer and more effective attitude than the parent doing their thinking for them.
Explain the specifics of association with lower quality people. Why are low aspirations bad? Why is antisocial behavior bad?
Appreciate and explain that we don't choose who we like, and we can't control who we are and aren't attracted to. That's nature working and she is quite powerful!
I know they don't want to hear "there are plenty of fish in the sea," but share your own experiences of how you held out for someone worthy, and it happened. (even if you have to make it up.)