Every age group has its normal transition threshold: That's what you see in "days until age up: X". Then they get 2 "bonus days" so you can plan a birthday party etc. That's the time when the age bar is sparkling and the sim hasn't aged up yet. You won't see "days until age up" on elders, you'll see "number of days lived" instead. However, you can still guess at the age progress because the age bar is still there. Elders are the only age group that can get more than the 2 bonus days everyone else gets. Having the active trait gives 10%, the long lived trait gives 25%.
The percentiles are based on the total number of days the Sim has lived, but it doesn't account for life resets. You can also drink cowplant essence for an additional 10 bonus days, and the wishing well from RG can get you an additional 5 bonus days if you wish for youthful vigor and get the right outcome.
Hope that helps!