I would say it varies in every case but I should imagine the sensible thing is to separate them when the first reaches the age of pubity - any time from the age of about 11 - room allowing.
I believe it is five years of age; I don't know for sure but I know of many cases of people who were in cramped accommodation waiting for a council flat, and the flat materialised when a child sharing with another child of the opposite sex turned five. So I gather that five years is when a local authority is under an obligation not to continue to house in the same room, siblings not of the same sex.