how old should a child be before they sleepover at a friends house?

I believe once you feel comfortable with the parents, and the child that your son or daughter will be visiting then it's fine to let them sleepover. I would always meet the parents first to see how they run there household or the way they control there child. If the child is comfortable staying with friends, I don't really think there needs to be a minimum age before they are allowed.

This should be up to the parents. Every child matures at a different rate. Some children at the age of 10 might act like they are 8, but others may act 12. I really don't think that the age should matter, it should be the maturity level of the child. I think that both sets of parents should talk it over and see if they can come an agreement. Test it and see if it works.

I imagine that like with everything else it depends on the child. And the friend, too. Some kids who are used to spending nights in other people's homes like family members would probably be more comfortable spending the night at a friend's at an earlier age.