It depends on how well you know the child and the parent and the family and their home.
It is not just by numeric age.
If a TRUST a parent/Mom/family and know them and have been to their home before, my children (who are 6 and 9) can be there without me.
BUT I do not leave my 6 year old son, at someone's house, without me... if I do not know or trust, the other parent/family/home.
I also go by my kids' cues.
I don't tell my son, for example, that he must be only dropped off... and then me leave. If he wants me to stay, I stay. But, I also ask the other Mom, IF she as a Host, wants the Moms there or doesn't mind of we just drop off.
I do stay, at another person's home/playdate, if I do not know the Mom or family, well.
My kids get together with other friends, often. At their or my home.
But not on week, nights. If on weekdays, it would be, only for a couple of hours RIGHT after school, BEFORE evening, and/or if my kids do not have a lot of homework, and/or if, we have time.
Weekdays, are very busy, for most Moms. Or on weekends.