If my mom is watching my baby, I'd trust her at 3 or 4 months. Anyone else... I'd say a year or two old enough so that if anything isn't right or they didn't have a good time they can tell mommy!

For my dad, whenever, he's watched her before, but in her whole year of life only twice overnight, and about four times for a few hours while I went to Walmart or got some errands done. Anyone else when she's able to talk/tells me she actually wants to stay the night with someone.

my daughter was a year old before she spent over night with anyone

When they are able to talk.