I've noticed this when switching between the types of lifespan, those being the likes of Short, Medium, Normal, Long and Epic. Upon switch, the new lifespan has the sim take the % mark of the lifespan that they were on.
As an example, I have a sim on Long which has 90 days for Adulthood to Elder. He is 32 days before Elder and if I switch to Epic and put it on 160 days, I find that he has 0 days until he becomes an Elder and ages up the next day. Is this avoidable?