J.M.
I think that the time a child becomes fully potty trained, and are able to clean themselves properly, they should start learning about privacy.
I know that my hubby never liked to shower with our 2yo DD too much... and he doesn't like her to see him naked any more. I think it's mostly because of the time she got curious while his back was turned, ran up behind him, and grabbed a handful of his junk! LMAO!!! Since then, he needs his privacy.