The law doesn't say anything about it.
The web site I usually give as reference for this is down at the moment - but it is Shelter. Google 'overcrowding shelter' and you will find it.
As a rule when the youngest is 10 he or she should not be sharing with a member of the opposite sex.
But the law disregards 'natural growth' for the purpose of overcrowding. Natural growth includes children getting older.
It is not considered unlawful for parents and their children to sleep in the same room on holiday, for example, and it is not in itself ulawful in the home.
If you are hoping to be rehoused because of overcrowding - opposite siblings having to share a bedroom - be aware that the number of habitable rooms, not just bedrooms, are taken into account. So this includes living rooms and large kitchens. It is a complicated formulae including age of children, number of habitable rooms and the sizes of the rooms.