Comment by pessimizer

Depends on if the rise is specifically in the fraction of the elderly population, or if it's in the fraction of the general population.

If the rise is in the fraction of the general population, then an increase in the proportion of the general population that was elderly would be expected to increase the numbers of a disease of the elderly, even if nothing was happening.

If there were a rise in the proportion of e.g. 80 year-olds that were developing Alzheimer's, that'd be a different story. Although you'd still have to ask yourself if some of the conditions that were increasing the proportion of the population that are 80 year-olds were disproportionately extending the life of people with a propensity for Alzheimer's.