My son is five, will be six in November. I'm pretty casual about it, like a lot of you. It makes me feel better to know how normal this is! I don't walk around naked most of the time but if I'm changing or getting out of the shower I'm not going to be super-careful about locking the door. I'm home with him and his toddler brother and it would be such a pain to always act as if I had adults visiting and be careful not to walk to the laundry room in a towel to get a clean shirt, etc. I remember showering with my mom when I was his age. I have no vivid memories of her being naked because it didn't stand out to me as weird, it was just natural and boring to me.
I do sometimes ask my son for privacy if I'm using the bathroom or changing but that's more because I'm a private person who likes a little quiet and space, it's no fun to be using the bathroom and have a 5-year-old burst in dancing around asking me to help him on Minecraft.