Elders just don't technically age at all. The bar goes up because after a set amount of time, they'll die, but before that, they have a random chance of just dropping dead each day.
That is wrong. Elders age just the same as every other sim. They have a certain minimum lifespan and once that is reached they have a certain daily chance to drop dead. And dropping dead is totally a function of aging. It's getting handled in the DayPassedCallback() in the AgingManager class.