But shortly after I was married (at 18 y/o) I joined the AF. We didn't know where we would end up stationed but we knew that with basic training and phase I & II of tech school that we'd be moving around a lot. We didn't want to enter a new lease/rental agreement so 3 days before I went in to basic training we moved in with my parents (no kids/pets). Then when I graduated basic and advanced far enough in tech school that I was allowed to move off base my wife joined me in Texas.

She was with them about 3 months. It was known the approximate amount of time going in and the purpose was just convenience.

Since then I've had a mother-in-law live with us after her divorce for 18 months (interesting stories...).

Also had a period of time where my parents (along with their dog) were trying to sell their home and my wife was having a difficult pregnancy and so they vacated their home and moved in with us for 6 months until after the baby was born. If they would have sold their home during that time they likely would have stayed living with us for an indeterminate amount of time.

I've had 2 sisters that both spent a year or more living with my parents with their kids (and husband for one of them).

First one had 2 kids and was divorced. Stayed with them until she was remarried and then the new husband stayed with them for about a month until they could get into a home for them all (that part with them all there to me is similar to the time my wife stayed with my parents while I was in basic training).

The other sister and her husband actually lived in his parents home for 6 years. While living there they adopted 3 kids. Then when his parents sold their home they moved in with my parents in a different state and stayed there for just over a year before they got their own home. They adopted another kid while living at my parents house.

I have an older brother also that moved home several times after leaving as an adult. Usually it was after getting out of jail and needing a soft landing spot until he could get back on his feet.

My parents have also had a nephew live with them. A grandchild (from the sister above that was divorced) once alone and later 2x with his wife and kids (he also lived with me once for 6 months). And one of my dad's sisters lived with them 2 different times.

Well before that as a young kid we had my mom's mom live with us for several years.

My grandparents on both sides often had at least one of their kids/grand kids living with them most of the time I can recollect. I just view it as what family does for family. I don't really feel like it is a millennial only thing - some families are just that way in that they "do" for their own - and have some intermingled in that "take" from others...