I used to have rental property in MD, and from what I remember, and I could be mistaken, there was no law that I was told of that said people had to be a certain age to have their own rooms. I think it's a management thing, but not a law. If a rental property owner is telling you it's a law, I'd ask them for a copy of the law, if they are dictating it, they should have it with them to show you.
The only thing I could find for MD - and maybe you can research the same place for DC, is that Social Services requests bedroom space for those wanting to foster a child and they CAN share a bedroom with same sex.
So maybe the person telling you that has it confused with requirements for foster care?