My sister would have happily shared a room with me until she was in her mid teens.

I, on the other hand, nagged for my own room from age about 6 and got it a little later (we had an unused "guest" room). DH hated sharing too, even with his brother at bording school (where they share rooms).

Guess what - it depends on the children and the space available! I had cousins who happily shared bedrooms until they moved out. They just changed which two shared and which one got her own room every few years. I've known cousins of different genders share for years happily too.