In Oklahoma I heard there is a law that requires a boy and girl from the same parents to have their own bedrooms. What is the ages that it is required? 7 year old girl and new born.

I don't believe there is any specific law but using common sense and knowing when the time to separate is necessary. I think it would depend on different things like is the 7 year old girl mature enough to know better than to bother the baby. Even with good intentions she may not realize her "strength" or that she could drop the baby if she is unsupervised and decides to bother with him.
Also she will get to an age where she wants her own room and privacy.

If you receive HUD, section 8, DSS then they should issue you an *upgrade* for a place with an extra bedroom. THEIR rule is (for the most part) 2 same sex to a bedroom.