If he is mentally mature enough for it why can't he move in with someone? You are probably not as mature as he is -and I quote your own signature here 'I am 18 years old but I act a lot younger'- that means he should be doing what is right for him rather than what is right for you.
Also he can...
I always thought Groose went off in search of new parts of Hyrule, possibly to go and start the Gerudo tribe. Most of Skylofts inhabitants would have come down. Those that remained would have mated with birds and though millennia of inbreeding devolved into the disgusting oocca race.
Link isnt a little boy in Suckward Sword. He can make up his own mind and it would be good for him to have chosen Ghirahim. He has a great tongue after all :ghirahim:
You don't need to be married to move in together.
Suckloft would offer no interest to someone like Link who had had such grand adventures.
She is a girl she can express her emotions however she likes. It is the mans job to suppress his emotions and Link is rarely emotional.
When you love someone like that, you do want to be with them. And that is reason enough to stay with them.
Plus Suckloft would be dull after exploring Hyrule. Link would have trouble going back to that life after his adventures.