Normally at what age kids should learn modesty?

You must have seen kids running around naked or not feeling embarrassed to remove or change clothes infront of others or coming out undressed or just in panty from shower. Well they are just innocent kids and I think its also cute and not offensive or unusual to anyone. But normally at what age kids really start feeling that its private and I should not show my body to parents or others? Actually the reason I am asking this is because I have sister who have 2 daughters. The older is 9 year old and younger is 6. When I went to my sister's home, I saw that my nieces were taking bath in shower and they came out like that only & then dry & dressed themselves after coming out. I & my husband was sitting there and their father (my BIL) was there but the girls weren't embarrassed of their father or my h. Infact the little angels became excited on seeing us and started laughing. We didn't felt anything weird or offensive but just it was itching in my mind that the younger is just 6 but I felt its weird for the older niece (9). Then this Diwali we all from our mom's side of family went out of station. My nieces wanted to go bathroom so car stopped in middle of road, my sister made them sit just near the car, I can understand it was night time and maybe she was scared going too far on side of road but there were gents in car and its not like anyone was like predator or looking but ain't it is sister's duty to make her girls learn a bit modesty. Then during sight seeing my older niece got hurt on her butt so my sister removed her underwear there itself and seeing where she got hurt. I pointed out this to her but she said that comeon they are just kids & all kids are like that till 10 and maybe you don't remember but we too weren't feeling any shame till 10-11. As far as I remember it wasn't later than 6. Btw I am recently married and shortly planning kids. Presently I am fond of reading parenting books so that it help to raise my own children appropriately in future. Maybe I offended my sister, but the reason I am posting this question because what I noticed about my niece was I found inappropriate and also I am just curious to know about it so that I do not make mistake for my own kids in future.

 

I don't say I am finding this in parenting books. I just mentioned in general that now that I am planning a kid I read some parenting books and not in the pretext of question I asked. I also don't say one should feel shame about their body. Kids shouldn't feel shame about their body or have difficulty in accepting their body. When I used the word shame or embarrassment I didn't mean feeling ashamed of one's body but I meant feeling aware about their own body and feeling uncomfortable changing or being undressed infront of others or parents/siblings/others of opposite sex (esp. girls). I know that if kids are made to feel shame about nudity then their focus & curiosity might increase on private parts. But here my question is that normally at what age kids learn modesty & feel on their own that its private and feel uncomfortable to change in front of others & also whether my sister is right or wrong or am I wrong to think like that? I don't think my sis or bil are dumb but maybe this is their way of thinking that they are just kids and maybe they being a parents don't feel anything weird about it. But I personally felt its weird esp. for a 9 year old girl infront of her father and I think its my sister's duty to make them learn modesty now that they are growing.