It is because the elderly are dying, same as they always been dying through out the ages. The older you get the more likely you are to die.

The current demographic trend is based on this assumption. It goes out the window the moment we start meaningfully reducing the probability of death.

> It goes out the window the moment we start meaningfully reducing the probability of death.

What does “Meaningfully reducing the probability of death” mean?

I thought death is certain. Do you mean pushing death further into to future, say by hundreds or thousands of years?

The probability of death is correlated to your chronological age. The older you are, the more likely you are to die.

Eliminating or reducing the probability of death correlated to chronological age means our average lifespan is likely to be thousand of years, but not unlimited since we will die to other causes, likely involving accidents and natural disaster.