Just two generations ago it was common for several children to share the same bed. These days, most children have their own rooms. Some mistakenly think the change is due to laws that dictate rules on children sharing a room, but the fact is, there are no laws dictating whether or not children can share the same room.
No real law for siblings
There are no United States Laws that dictate an age that siblings of any sex can share a room. As long as there are no complaints, parents can make reasonable decisions as to who can share what room in their household.
Apartment complexes and public housing can have rules for tenants that dictate how many bedrooms are required based on the number of people in the family. This rule is generally two people per bedroom. The complex also may require that children be separated by sex.
Child Protection Agencies
Child protection agencies can make judgments about children sharing rooms, based on the situation. The rule of thumb, in most cases, is for children to be separated by sex before the oldest child reaches puberty. In some cases, they will require children over five be separated by sex.
Because foster children are wards of the state, there will be laws that dictate the children sharing a bedroom. These laws may vary by state. For example, in Ohio you can have four children in a bedroom as long as there is room for personal belongings, clothing and the room is safe and comfortable. Children over five may only share a room with same-sex children, and foster children over one may not share a room with an adult.