It is weird. The first time my son brought his girlfriend home they spent most of the time in his bedroom (both aged 14). I was unsure how to feel about it. I think (probably naively) that they were happy to have some space away from us and on their own.
It is the time to have frank, open conversations about everything (if you have not had them already).
They are still together (he turned 16 today) and they spend most of their time in his bedroom (apart from when they want feeding).