1) it is up to you and what age you feel okay allowing it.
2) Sleepovers can be at YOUR home... or your child's friend's home.
3) For me at least, I ONLY allow my daughter (she is the oldest and is 10 year old), to go to a GOOD friend's house, in which I ALSO KNOW THE parents/Mom/family/siblings, very well.
4) My youngest is 6. Sure he asks if he can go on sleepovers too like his big sister. We tell him No. He is too young. He understands.
5) My daughter, had her first sleepover when she was turning 8. But it was for her Birthday and it was at OUR home. The invited was just like 4-5 other girls who were GOOD consistent friends of her's... and which, we ALSO knew the friends and their family, well.
Then... from when she was about 9... she went to her GOOD friend's homes, for sleepovers. These again, are her GOOD friends which we also know well and their family... and which we have ALSO been to their home too... socially. So we know, their home environment.
6) I personally... would NOT send my kids to a play-date or sleepover... if I do not know, the parents or what their home is. Not at all. AND if it is a play-date... I only would send my kids to another person's home... if I also had been there before and KNOW, the Mom/family and what their level of responsibility is and their level of supervision.
ie: my daughter had been invited to a girl's home before. But I said NO. Why? Because, the Mom is known to NOT supervise her own child very well, much less when there are friends over. I know this, personally. And my daughter even, has also said "Mommy, Sally's Mom isn't very responsible... she just lets her kid go wherever and doesn't even know...she doesn't even watch them when they are in the pool...."
Then, I also know a woman, who's niece went to a sleepover party. They were about Middle School aged. She said that, in the middle of the night her Niece called home to want to be picked up. Why? Because, during the evening other people came over that were friends with those parents. And during the night, one of the men... went to where the girls were sleeping (which was in a den like room). Well, that man started to touch, the Niece. So as you can imagine... this was traumatizing. For all. And a case for the Police.
USE your Mommy radar.
Don't just let... your child go to a sleep over or for a play-date if you have any reservations about it, even if... your child is "pressuring" you or saying "all my friends do it, why can't I?"
With ANY sleepover that my daughter has, I TELL the parents and their child, that they can use our phone to call home or the parents can call me ANY TIME OF NIGHT, even if it just to say goodnight.
And they can drop by. Anytime. There is nothing to hide.
AND when my daughter goes to a friend's home... her friend's parents are the same way. Accessible, no matter what hour.