At first I thought it was just a general mammal thing, and it probably is, but the question really is why only some mammals have evolved to survive after the reproductional age.

The article briefly mentions food availability as a key, and there are many other external causes, so it's still not understood how or if longevity is inherited. In humans it's generelly accepted that a long lifespan is partly inherited and partly due to society.

The similarities to other species is interesting to confirm, because it shows that the construction of a society can maybe influence the genetic evolution. In short, it's (maybe) a sort of indirect feedback loop, where if a society can afford to have old individuals, it will also evolve to more individuals getting old. I realise that evolution itself can not do this alone, but it can apparently take advantage of happening in a society that enables this.

Anyway that's just my take on it, I'm not a biologist.