I wonder if the benefit of interaction between young and old works both ways. Where I live now and in many countries, older people are much more integrated into society than in the US (e.g. extended families living together, grandmas cooking food around the market, grandparents monitoring the younger kids while the parents are working).
I used to hardly ever interact with old people, but now I do every day. Some of the friendliest people in my neighborhood include a very elderly lady who counts the money and makes change where I buy coffee every morning (she has tremors, and has maybe had a stroke, but still has a function in society) and another older women who rides in on a bicycle carrying a large basket of really excellent fruit. I definitely think having a purpose, even if it's because poverty requires everyone to pitch in regardless of their limited abilities, does tend to keep people happier and more social as they age.