Well, yeah. Me, my wife, and my kid live with my dad. I'm almost 50.
Mind you, I bought the house from him. But the whole "can't have a family home" thing where you have to live separate from parents or grandparents to be an adult is utter bullshit. It is often easier to navigate the interpersonal stuff when it's the classic nuclear family and the kids move out to start their own, just because relationships and the work of them is exponential based on the number of people and the number of relationships between them. If you're the parent and the landlord to an adult offspring, that's two complicating factors in making things work peacefully and (hopefully) happily. Add in another generation, especially when grandparents are part of the child rearing, and shit can get messy fast.
We make it work by the framework of: my house, our home, your room.
The house itself is mine, I have final say in structural changes, repairs, etc, because I'm the one on the hook for any legal issues that derive from such. But the running of the household is by consensus of the adults, and input from the kid, with agreed on boundaries. Within those boundaries, if you're in your own room, you do what you want. The kid is aware of what the boundaries are, and that they won't be changing when they become an adult, and they'll have the freedom of choice to stay or head out, knowing there's a safety net here they can rely on.
They ever have kids, those kids would have the same choice.
Yeah, a house can only hold so many people before it becomes a chaos that isn't bearable. No matter how big the house, that remains true. But a family home is still a very valid and good choice where life makes it useful/necessary.
Shit, on my end, if the kid stays here until they're in their fifties, I'm happy as hell, as long as they're here because it works for them. They'll be inheriting the place if I get it paid off before I die anyway.
I moved back here as a temporary thing in my late twenties. Left the city I had been working in and was looking for a place of my own. My best friend came with me, and when my mom finally moved out post divorce, it just kinda worked until I had to buy the place. After that, it still worked, and the people involved have changed a few times, but there's this wonderful sense of connection and security knowing that we all have a place to be if we want it.