[scarmig]

Another point to indicate that the elderly do contribute back: if the elderly were a net negative while we evolved (which involved a lot more physical labor than typical work does today), there'd be selection pressures against the genes that confer longevity. And yet we've evolved in a way that the typical person will live long past the age of reproductive viability.

Concretely, humans aren't that much smarter than chimps. We did reach a point, however, where knowledge can be transferred and processed efficiently across individuals ("society"). The elderly play a net positive role in facilitating and maintaining these knowledge transfer networks, particularly to their kin.