What age should your child be able to sleep in the same room as their boyfriend?

My 17 year old has just finished high school (secondary school) and will go to university next year. She's had a boyfriend for 4 years and is wondering when they can share a bed. Just wondering what other peoples opinions are? I think I will allow it after her exams when she will have finished high school forever as I trust her and him and would rather have them being safe under my room than a park or somewhere like that. Her boyfriend is 18 and his mother is okay with it.