gotta agree with RhiChiChi on this one. Although I would vary it with taking into consideration the age of the teenager. A 13yr or 14yr old, there is definite responsibility. When you are talking about 17 and 18yr olds however, the lines start to blur abit.

The problem is that no matter what age your kid is, it is impossible to know whether the parents 'parent' and supervise in a way you would feel is appropriate anyway. I do not like some of my 'in laws' looking after my kids, because their idea of supervision does not hold up to my standard. And these are young children, I would be extremely hestitant to trust them with a teenager for the same reason (only little kids, might run off or hurt themselves, whilst teenagers can be ALOT more creative and get up to all sorts of things - at least I did when I was a teenager).