Just as with any other "roomies," parents and grown children ought to establish house rules. A mother/daughter house would also be useful; 2 kitchens, 2 baths, 2 entrances. Great resale value too!
As for turning the music down I recommend headphones. :)
Incidentally, parents should look ahead and consider that one day they may have to downsize to a place with fewer rooms, everything on one level and a bathroom that can accommodate a wheelchair. They should consider making that move before they need to, and keep in mind the possibility of sharing it with the younger generation. (That is, if they WANT to avoid Shady Pines.) Find something while they can still qualify for a mortgage.
When hubby and I bought our last house almost 20 years ago that is stuff we thought of. We looked at location and accessibility. I never would have thought of it but after hubby’s dad had bad neck bones he slept in a hospital bed in their living room for over a month after surgery. Couldn’t make it up the stairs to the bedroom. They moved not long after that and it made me really assess the houses we looked at when we sold and bought again.
Now I’m so glad we did since hubby’s knees and back iaew so bad he sometimes has a hard time going from the den to the master bath on the other side of the house.
Daughter has a 2 story and when I watch the kids the stairs kill me. Lol