When I was a kid I remember playing doctors and nurses, but there was no exploring of any naughty bits. Checking heartbeats, that sort of boring stuff, was all we did. I remember my mum freaking out and telling us we weren't allowed to play doctors and nurses and never realizing what the issue was until a good 20 years later.

My son comes home from school with candy and gifts from one of the girls at school. They are both 6 years old. She also writes him love letters. He is, of course, completely naive about this sort of thing and treats girls his own age the same as he treats boys.