There's a huge difference between nudity in the neutral sense of "no clothes on" and erotica (which may or may not involve nudity, but usually does).

My kids see nudity all the time. They see me and their mum nude in the mornings when we're getting dressed, they see people nude in the changing rooms at the swimming pool, they undoubtedly see other kids nude at nursery in the course of the day. That's all normal and healthy.

That's not the same as letting my 3 year old watch porn.

Porn is a complex subject even for adults, and absolutely needs an adult perspective to contextualise it, understand it, and potentially recognise when something about it is seriously wrong. This is something that is perfectly reasonable to limit to adults.