I don't know if I'd personally do this, but I had two older brothers who shared a bedroom, and then I had the other room. My parents had another baby boy when I was 6, and his crib went in my room. He stayed in there with me until he was about 3, I think (which made me around 9). That was when the oldest brother left the house & he could take his spot the 'boys' room.
As a child, I don't remember ever feeling weird about it.
Now that I'm a parent, I'd probably say age 3 or so would be my cut-off (for which ever sibling is younger). The parenting books I have say that's the age when it's best to try and stop the naked-parent-viewing as well, so I kinda put those in the same basket. It seems like around then would be the age kids would start remembering things, such as watching their big sister changing, or naked out of the shower, which might make life wierd for them.