Well I can say from my experience that it's generally a bad idea. But like many have said you can't stop them in a basic sense unless you put her under arrest, but that would be pointless too. Trust me, my parents tried that with me I did what I had to, to get out. I was 14 with my first boyfriend and he was 19 and I can tell you that things could have easily gotten out of hand. Not that they didn't in a way. My parents flipped when they found out how old he was and even threatened to send me to bootcamp. They thought I was out of control, but they didn't realize I wasn't having sex, even though I could have been. They didn't realize I wasn't drinking or doing drugs, but I could have been. My friends, my cousins, almost every other teen I knew was having sex and doing drugs, but I wasn't because I knew I wasn't ready. Some of the teens I knew at that time were even having babies and dropping out of school.
So you have to ask yourself, if it's your daughter we're talking about, what have you taught her over the years? What kind of morals have you instilled in her? How much do you trust her? Do you know the boy or anything about him? Have you met his parents? Each child is different, so take that into account.
If you are the teen asking because your parents won't let you date. Know that they are doing it for your benefit. You might be mad at them now, but you'll thank them later. Cause if the boy is worth it, he'll wait for you. Don't rush into things and try to grow up too quickly or you might just get what you ask for.