What's the right age to let your kids start dating?

For me and my husband at age 18 my daughter is allowed to have a boyfriend, we even warned her that if she is going to enter into a relationship she will stop schooling, but its not effective, because we found out that she enter into a relationship many times secretely from age 14 up to the present when she is 15, what we did is talk to the boys and explained and informed them that we did not allow our daughter to have a boyfriend and go on dating, and we warned them if they took our daughter, we will sue them. Luckily, they end up breaking up our daughter, I know it looks like a harse way but it is also a way to test if their intention is real.So far our daughter had a bf through chatting in messenger, and not in person. If we are to choose after finishing college is the ideal time to have a bf but, it is not under our control because they have their own decision to make, what we just can do is give her guidance and support.