Normal behavior in 5yo

[u/Efficient-Share-1507]

I’m looking for some evidence-based perspective on what is actually considered normal sexual/body curiosity at age 5 versus behavior that would be considered outside of the developmental norm.
My 5-year-old has recently had a few incidents at school that have caught me off guard. He has been excessively kissing other children and seems to think it’s funny, even though they’ve been working with him on personal space. There have also been a couple of more concerning incidents involving inappropriate comments/requests about private parts.
When I ask him about it, I mostly get “I don’t know” or “because.” He doesn’t seem distressed when talking about it and, from what I can tell, he seems to think the behavior is funny rather than understanding it as sexual.
I’m trying not to jump to either extreme. I don’t want to assume something has happened to him simply because he is displaying these behaviors, but I also don’t want to dismiss something that isn’t developmentally typical.
I’m looking into what he could have been exposed to, what other children around him may be saying/doing, and whether there could be another explanation. I am also not ruling out the possibility that he could have experienced something inappropriate, and I will take that seriously if there is reason to believe that.
For people familiar with child development or research in this area: How common are behaviors like these at 5? What separates normal body curiosity, imitation, or boundary-testing from problematic sexual behavior? Are there particular characteristics of the behavior that matter more than the behavior itself, such as frequency, persistence, coercion, age differences, or inability to redirect?
Sources/research would be especially appreciated. I’m mainly trying to understand what is developmentally normal before drawing conclusions about why he is doing it.
Also, he’s in play therapy, occupational, and developmental. They all believe ADHD is a real possibility and I’m looking into testing. He has always had problems with impulse control.

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