Fostering an adult

I'm a "single" women as in I'm married. I do have a boyfriend who doesn't live with me. I got my fostering license 8 months ago and since I've had 3 kids. A brother and a sister who were 4 and 8 and a baby who was 4 months. The 4 month old is back in his parents custody. I have just been waiting. A get a call asking if I would be okay fostering a 20 year old! I forgot that you can stay in foster care until 21. The caseworker seemed pretty desperate and said he will be aged out in less than a year. What I was told and he was removed from his home at 5 from abuse that almost killed him, then was in the foster system and moved around until he was adopted at 11 and then removed from his adoptive family at 15 because they were abusing him and has just been in foster care since. He ages out in June of 23. He's a cancer like me lol. I agreed. My boyfriend isn't happy at all and doesn't like the idea of me having a young man walked around the house. I'm 41 and even thinking of a 20 year old that way makes me want to vomit! But I'm going from just having an infant and a grown man and I have no experience fostering teens. Let alone adults and the rooms for my foster children are kidish. I'm having a small panic attack because even though I took the classes I'm nervous about having to foster am actual adult because they obviously think different than small children. IDK if anyone was in foster care until 21 but did anything help you? Any foster parents I'll take advice! He will be here in a few hours.