What is the age to move out of parent’s house legally?

In Illinois my son who is 16 is dating a 17 yr old girl. They have been dating for about 3 years always supervised but his girlfriend has come to our house showing classic signs of child abuse. IE Black eyes, burns that look like they come from a car cigarette lighter, Bruises on arms and legs. I have asked the girl many times and she has told me both nothing was happening or yes and she was seeing DCFS. Well at the age of 17 she wants to know if she could move in with us till she graduates high school in 2 1/2 more years. My son will graduate before her. We have a spare room downstairs where she could sleep and he sleeps upstairs. I stay up late most nights 1-3 A.M so can assure you there would be no hanky panky. I don't think she would turn her parents in if I did call the cops and got them involved and I am already treading on thin ice as something has to have been said as she was not allowed to come down this summer. (I asked a couple people who knew this girl if they knew if she was getting abused). She had to go to my other sons Girlfriends house and spend the night to come over for our 4th of July and Thanksgiving BBQ's.