Thanks for this suggestion.

I also suggested to look for such a phenomenon in vampire bats, and other reciprocating species. Do bats stop co-operating after a certain age? (Or do other bats stop co-operating with them?)

In my experience, old people are LESS likely to defect in Prisoner's dilemma, as judged by real-life instances. And other people are less likely to defect when interacting with them. This fact is worthy of some explanation, as it's not what the basic theory of reciprocal altruism would predict.

The best explanation I've heard so far on the thread is that it is because of reputation post-mortem affecting relatives. This requires a social context where the "sins of the father are visited on the son" (to quote Randaly's example).