Most people do not live with their grandparents. They get shipped out to senior living facilities or they live by themselves. The wisdom they could share and the joy they could get from seeing their children are miniscule compared to the resources and the space they take up, which could have all gone towards the younger generations. I suppose it would also have to do with the age that's on my mind, and I would probably peg it around 65 and older when they have effectively retired from the workforce. From then on, I believe living longer than that would be causing more problems for the society as a whole. As for the wisdom, it's not a given that all old people have great wisdom to share that which cannot be obtained via the internet, and I would also argue that their cognitive and physical decline will also get in the way of effectively sharing their wisdom. There's also a difference between living healthy to raise the quality of life and just so happening to live long and living with an intent to live long and expending all assets to keep the life going. The hospitals insist on extending the life of the patient in order to make profit from drugs and procedures, but I doubt they have the patient's quality of life in mind.