A Question For Mom's With Sons?

Okay, my older sister has a five-year-old son and I was just wondering something. If she's in the bathroom taking a bath he'll just walk in and do something like go to the bathroom or ask her a question or if she's changing he'll just walk in no problem. My other sister has a two-year-old son and it's the same thing, I was just wondering if it's normal for mom's with sons to do that and if so what age does it stop? I'm asking this because I was watching Malcolm In The Middle and in one of the episodes she's walking around the house while her bra dries without a shirt and while I know ti's jut a show I was wondering if it happens in real life. .