I disagree with some of the comments that because she's in college at that age means she must be more mature. In some cases, I've seen it to be just the opposite. I had a friend who finished high school at 17- and already has an associate degree by then. Her college grad actually took place a few days before her high school one. But while she was a very hard worker when it came to school and such, she was lagging behind in some other areas - even behind the kids who were only a year older than her. There are very different types of maturity and just because you've mastered one doesn't mean that you are above average in others...
That said, I'd be careful - but also be realistic. You don't want to be like my cousins mom....
My cousin turned 18 in Dec. She's been with the same guy for 3 years - but mom doesn't like him because of something dumb he did when they first got together. But mom was not aware of this. So they were going to get her on birth control when she turned 18 (apparently mom thought that was some magic number? I don't know.) Just before the appt, mom found out that Cuz was still seeing the BF - and refused to take her the the appt. since mom's insurance was covering it (she was still in HS), Cuz had no say. Few weeks later, guess who's pregnant? That was just stupid of her mother. The bottom line is if she wants to have sex, she's going to do it regardless of any rules you may make. But that doesn't mean that you have to allow it under your roof. But I really don't think you can control whether she goes over to his house or not. If she really wants to, she's going to find a way. You can discourage it, but just as somebody said with regards to sex, I wouldn't assume that it doesn't happen