For the crass materialism exhibited by some wealthy retirees, the classic inter-generational relationships are not necessarily a panecea. They seem to work well in agricultural settings, from what I've seen with the Amish, but in more urban setting they can be problematic, since social and economic change occurs too quickly for for the retirees property to be much good, outside of its monetary value - Dad's high end slide rule is cool, but useless. Further, when social and technological change is fast, the skills knowledge, etc of retirees become less valuable in child rearing. I guess you might argue that this is because of how industry compartmentalizes technical skills, and treats skilled workers as disposable, but that's another argument entirely.
the other thing not mentioned here is the significant expense of health care for the elderly. The cost of the necessities for survival go up as we age, if we include things like medicine and health care.
So yes, If we were to all live like the Amish, this would work. But there are many downsides with Amish style living that I won't go into here.