During the Depression it was normal for people well into their 40's to live with their parents, and for single women to live with their parents.
Read "Anybody Can Do Anything" by Betty MacDonald.
She moved back home with her mom, bringing her 2 girls, when her marriage fell apart.
Her sister was also living there, as well as the minor sister.
It was just normal, they all helped one another, pooled their money and resources, babysat for one another, etc.
It made things easier for everyone.
It wasn't until later (like 60's onward) that the idea that you want your own space if you're not married came around.
And sometimes I wonder if that's a misguided concept.
I know so many single-moms who would benefit from living with family, either sisters or parents or something, so that they had more support for their kids.
Maybe it's my perspective as well, since we live with my parents.
I want my own place, don't get me wrong, but my parents are 76 and 80 and their health is failing.
By us living here, we pay rent (so taxes and things are easier for them) and make it easier for them to live in their house.
My dad has fallen a few times and we've had to help him up, my mom injured her knee 2 weeks ago and couldn't walk so I was here to do her laundry and help with meals, etc.
And it's not always just the very elderly who may benefit, my husband's step-father has been diagnosed with stage 4 cancer and may not have much time left.
My husband hates the idea that his mom is having to deal with this alone basically, and wishes we were there to help her out, do laundry, clean, etc, so they could focus more on fighting this.
So long as the parents and the adult child (or grandchild) don't have a problem and are getting along and have basic respect and such, then I think there's no real age where it's inappropriate anymore.
Add: I should also point out that when I have to take my kids to the hospital, I don't have to bring EVERYONE.
My mom can be told and she'll keep an eye on whoever isn't coming so I can focus on the one needing that emergency care, and every emergency seems to happen while my husband is at work.
Or if someone is sick and I have to run to the store to get something to help them then she can help me there.
If she wants to go to the store and we're going, we save gas as well as help my parents carry things in, if I'm out and they need their medicine picked up but are feeling sick then I can pick up their medicine... the benefits for both of us have really paid off over the years.
Add: Just to point it out, I disagree with the 'if they move out and have to move back home they're better off' statement.
The people in my family who rush to move out and then fall flat on their face and find themselves back home almost always aren't functional adults.
If they fail because they lose their job, unexpected pregnancy, something more out of their control that's a bit different.