I think that my tolerance would increase with the age, maturity, capability, and performance of the kidult.   Think a multigenerational home concept.

My parents keep presenting the option of DW and I retiring early and moving in with them.  DW is very interested. As am I on some level.  Mom, dad, DW, and I all get along very well.  Their home is 5Kft^2 with 6Br 4full Baths and 3 car garage.  They built it as their retirement home and the extended family home base while we were overseas as was my brother.  This is the model that we have used for 3 generations.  Whoever is stateside is the address of record for the Expats out globe trotting.  Mom wanted everyone to have their own room for family gatherings.  That happened fairly frequently when the GKids were all younger. Now that they are all kidults, not so much.  My brother and his DW have a home in the same area, as does my niece and her DH.  So, the only people staying with my parents regularly are DW and me, and our spawn (SS-31) when there is a clan gathering.  Nephew and his DW usuall either stay with his parents or with my niece. Youngest nephew stays with his parents.

I would be fine with aging parents living with us, though in the case that my parents keep broaching, it would be us living with them.  Though we are the only one capable of supporting my MIL, she will not leave her small home town which is where DW's three younger sibs and their kids all live as well.  DW will not send money there. Period. Dot.

As for SS-31 living with us.  Not out of the question nor is it particularly off-puting.  It may happen at some point after he retires from the Military is just under 7years.  Though his cat cannot come with him. I am deathly alergic to cats and dogs. Except for the haired or bald versions.  Furred, nope. I would be in constant respiratory distress or in an eternal sneezegasm if a cat or furred K-9moved in.  I love them, I just can't have them in my home.

Depending on the property, I would definitely consider a granny pod/small home in the garden for a mostly independent aging parent or two, or responsible kidult. Or as a guest house/rental/Vrbo/Air BnB. 

It would be a cold day in H3ll before my SS would ever be allowed to move in.  We're no contact, so there is that.  My bios, however, they would be welcome.  They are both teens right now, daughter is about to graduate high school and will be 18 this summer.  If they were adults I'd expect them to contribute to the household expenses and help keep the place clean.  I imagine they'd be ready to move back out on their own if they're as much like me as I think they are.  

We sealed our revolving door shut w/ Gorilla Glue. It was one of mine who caused it but it applied to all 3. Of course, two are married (mine) and one engaged (his) and of those three, two own homes (my oldest, his son) and one doesn't yet (my youngest) but if there were to be an emergency of course we'd clear the crap off the guest room bed. Keeping in mind "Guest" room means guest not life time resident. I'm a bitch not an ogre. If there was dire need we'd slap on our super hero capes and offer our assistance. 

Honestly.. I think that most bio parents would allow their kids to live with them.. at least temporarily under some circumstances.

I don't think you have bio kids.. (neither do I actually).. so it's probably a bit more of a stretch to think about if we haven't.

1.  Child escaping an abusive relationship.  If I had the room I would provide emergency housing.. maybe for 6 months to a year.. or if no room.. but had means.. financially help my child secure housing.

2.  Child a victim to a natural disaster/catastrophic loss...  There is a flood, fire, earthquake, hurricane and their primary home is destroyed.. I would provide immediate temporary shelter.. and it would depend on what the timeframe and resources they had from insurance were.. whether it would need to be long term.. let them park a camper on the lot while they rebuild for example.

3.  Other sudden issue.. child's spouse dies and leaves them without a home in some immediate fashion and they need somewhere to be immediately.. again.. like the abuse situation.. a short defined time.

4.  If I were older and was starting to need assistance and there would be mutual benefit I would consider it.. 

5.  Child is between defined housing windows... buys a home.. but lease is up before closing can happen.. I would allow them to stay in that interrim.

Of course, all this is predicated on the assumption that if I'm single at the time.. it's my choice entirely, but if I have a spouse that is not related to my child.. that they should have some say.  I might push that the urgent and immediate assistance in a crisis be a "this is not a discussion I'm informing you"  kind of thing.  (Daughter shows up on doorstep with a black eye.. and needs a place to be tonight as she leaves that creep of a boyfriend.. my partner better not tell me it's not possible.. to be honest.. but I also imagine that my daughter would have been raised to respect my partner... like my skids were raised by their dad.. so I wouldn't deny him the ability to give one of his kids shelter.. obviously there could be some situations that are more extreme).

When would I not?  Probably if my child was involved in criminal activity or addiction issues.. I would not allow that in my home.  

My DH and I agreed a long time ago that kids could stay living with us after 18 and graduated as long as:

1. They were a full-time student in college.
2. They were a part time student in college with a part time job.
3. They worked a full-time job with part time college.
4. They worked a full-time job and were saving up to move out.

This pertained to all the kids, even Spawn. My BS did move back in with us right as Covid was starting to ramp up, he lived with us for three years while he saved up for his own place, and he did pay us rent and worked a full-time job.

It would depend entirely on the people involved. Not gonna lie, i would tolerate my own kids or parents looong before i would tolerate someone else's. That's what makes being a stepparent or daughter/son-in-law to those needing help so hard. You are expected to treat them like your own but they aren't.