UK Landlord Forum / Life

I have a young couple wanting to rent a cottage through our agency and one of them is 20 and working  and his girlfriend is a 17 yr old student.. who will be 18,  4 weeks after they move in, her parents are very happy to stand as guarantor and we  will will draw up a guarantors agreement accordingly, I was just asking if the tenancy  will all be legally binding if anything was to go wrong later on in the tenancy , as she signed it before she was 18 im sure im worrying unnecessarily  but just want to make sure I get it right as we dont usually have 17 year olds who want to rent .. many thanks