This is normal, starting around 8. I would not be concerned. We parents normally think our children are too young for any type of sex education until after they have learned (wrong or right) about it somewhere else, but 8 is not too young to start simple. You can do it or wait for sex Ed in his school like most parents these days. But he will be curious until he becomes comfortable about it. At 10 or 11 it will probably be you he is interested in seeing if that is your worry.
I grew up in an age of open showers, so I raised my boys the same way. I never worried about them being curious with some other adult.