DD is 7 years old, and DS is 5. They have shared a room since DS moved out of ours and seem to love it.
However, I'm very conscious of two things - firstly, that being different genders, they will need privacy at some point - and secondly, that we have no way of moving for the foreseeable. 
When do you think the maximum age for children of different genders to part company is? They turn 8 and 6 in the spring.