DD's school friends started so early - many had done sleepovers by pre-k, as buddy time but also as a function of a close knit community trading opportunities for date nights since a lot of us don't have family in the area.
She went to her first sleepover at 4 or 5 at her best friend's house, and fast forward 5 years - we've continued trading off every couple of months and it's been great. Other kids started sleeping over with us and dd sleeping over with them more like 1st and 2nd grade, all with a handful of families we know well and trust.
By now, the kids are regulars at each other's houses and it's not a big deal to be there, so the bonus for me when I host is that they don't stay up all night like I used to as an older kid on my first sleepovers. They play, they eat dinner and watch a movie, then they brush their teeth and go to bed, maybe read and chat for awhile, and then quiet down to sleep. It's more like having surrogate nieces over.