My eldest was 7 on her first sleepover. I did know the mum quite well but had never been to their house. I had only met the father a few times. I now know them much better (still havent been to their house though
) and found out a good friend knows the father very very well. He is also a police officer hich, rightly or wrongly, put my mind at ease a lot the first time.
No matter what age, I would have to know the parents and child and I trust my instincts on these things. I also never leave my kids at a party unless it is an organised party with professional host, I am good friends with a parent staying or I know the parents. Once they hit the teen years, then I don't think I will be as picky but I would still have to at least have met the parents a few times.