[waps]

Lifespan is a function of organism size (ironically measured in weight, mostly). The bigger an organism, the longer it's lifespan. Keeping in mind normal human lifespan would be about 30-35 years, you find that many animals of similar size have a natural lifespan within a factor 2 or so of a formula that also applies to humans. Normal whales, for instance, have a natural lifespan of about 70 years.

That this formula works would seem to indicate that you are right. Death is natural, but it's "planned". The easiest way for such a formula to work would be that your genes somehow contain a death clock.

But there are multiple death clocks. One limits number of cell divisions. There is another one known that limits the amount of energy that can pass through a cell, after which it will kill itself. There are various others, one that kills the cell if it isn't deactivated on a regular basis (presumably meant as a check on DNA integrity), one that is triggered from the outside of the cell, ... the list goes on.

[ehvatum]

Size alone has limited predictive power for lifespan. Consider the example of the naked mole-rat (30-35g, 31 yr) vs Norwegian rat (450g, 3 yr). Naked mole-rats dwell in environments shielded from predation, whereas no R. norvegicus in the wild could possibly hope to avoid attrition for 10 years, even if its ageing process allowed 20 years of healthspan, let alone 30.