Yes, but that is quite a different situation than what is being discussed here. For example, I foresee myself taking in my MIL at some point as her health is failing and will soon no longer be able to live independently. But my wife and I will be masters of the house and she will be under our supervision and care. I will be paying the bills and providing the shelter.
With many of these millennials, it is they who are dependent and "under the thumb" so to speak of the parents. They might be 25 or even 30 years old but they pretty much have to sneak in the door when they come in late and sheepishly explain where they have been if they find their mother waiting up for them in a bathrobe. It is they who have to be told to "turn the music" down and get nagged about piles of dirty laundry (that they expect their mothers to do) and un-rinsed dishes in the sink.
That's embarrassing and humiliating and as a result, they are trapped in a never-ending stage of mental adolescence.
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.