My dad lived with us for a few months when we moved him across country and we we're all miserable from his drinking, smoking related chronic bronchitis, and shouting in his sleep. But our 25 year old DS, DH and I, we all get along great and do music activities together. DS would like to work full time and get benefits, but in the meantime his two professional part time jobs leave him pretty satisfied with his life, and we all contribute to household functioning.
So DS gets to build his nest egg at minimal cost to us and himself. I can't help seeing this as a win-win. Our large enough house gives all of us the privacy we need.
My dad grew up in a 3 generation rural farm household. My next door neighbors growing up--the son stayed in the house and lives in it still. I had a coworker living with her parents and sister--she is now 40. My husband's best friend had the same situation. He lived years under his parents' roof, then bought a condo a mile away, and moved back into the home of his childhood and is there now, at age 60. That paid for home is worth $1M in the heart of Silicon Valley. Financially he is way ahead.
As long as DS feels he is living his life his way, I'm ok with it. It works.
All this means we live the life we want to live, regardless what life is "supposed" to be, whatever that means. I cried when he went off to college. Little did I know that six years later we would not be empty nesters. He is still here, and it does not bother me. As a mom, I am really selfishly content with it. He is living the same life he would otherwise live, just rent free.
Also, he knows about our financial situation. There is no point for a clean living good person such as DS to not know he is secure, and to learn about working and investing. He saw us take care of my parents through illness and death, saw my mom's psychosis from steroids and later from lung cancer, our taking care of dad while working full time and still figuring out how to devote time to him in high school. Because we kept him involved in this, he knows we will likely give him some difficult times at the end of our lives. I mention it occasionally, and have apologized in advance for those inevitable challenges.
He does kind things for us and with us. He remains part of our lives. I know he will launch himself when he is ready. It is a topic of earnest and honest discussion in our household.
My niece hated her parents and moved out after college. Three years later, she dumped the boyfriend, works in a dream job, but lived 15 minutes from home, worked a second job alongside her dad at a ski resort this winter, and has taken up mountain climbing as a hobby. She is grateful to be near her parents.
I'm thinking the nuclear family may be a 20th century anomaly.