I had sleepovers very young, I remember some about 4ish. But that was my parents out for the night at friends' house, they had children our age, we just all got put to bed together, the parents got drunk, staggered home/went to bed and the children woke up and went home in the morning.

I have allowed Liebs to have his bestie over one night in the hols now, he is 5. But only that one child. And not again until he is a LOT older after that one night! NightMARE more like. Because his bestie's mother needed childcare the next day from about 6.30-7am ish and Liebs wanted his bestie round to visit, so I offered a sleepover. I'm sure I would take the same chance if I needed it. I wouldn't do a school weekend as if the children didn't sleep there isn't enough recovery time, IMO, for school on Monday.