I play with aging off, so literally everything is "when I am ready to."
That being said, in all these years of the game, I have never aged up any sims to Elder... no, wait, that's not quite true. I aged up Geoffrey Landgraab to an Elder while Nancy is still an adult. Even though it's canon, or at least semi-canon, that they are the same age, I wanted her to seem a bit more like a gold-digger. Or, at least like she likes having her doting and naive older husband wrapped around her finger while she does whatever she wants.
But I digress. Most of my adults will eventually be aged up to Elder when their kids start families of their own.
The one exception is that I started a few households for Cottage Living where the eventual goal is to have them be elders in the same timeline as my other sims, but I wanted them to raise their kids rather than just creating the children in CAS. So I'm playing those households* sort of on a separate timeline, mostly just interacting with each other. I've set that aside for a while, but the goal is to have them play through a couple of generations, starting as young adults, and then reintegrate them into my main timeline. It's basically a save within a save.
*Most of them are within my Crumplebottom-Goth family tree. I created a bunch of Crumplebottom siblings, and then added Cornelia (Crumplebottom) Goth to Mortimer and Bella's household, and set her as Mortimer's mom. (She's already an elder, so that she can fit into that household's timeline.) Cornelia also has a brother and sister (Rupert and Amelia). I wanted to make Agnes their sister as well, but I've never been able to figure out a way to add her or cousin Agatha to the family without affecting her NPC status or messing up the stall. Anyway, Rupert and his wife Winifred are currently newly married YAs, and they are going to raise twin daughters (Pandora and Primrose), who will eventually both get married and each have one child of their own. Sadly, a tragic cowshed fire will eventually take the lives of all adults (parents and grandparents) in Rupert's branch of the family, leaving young Tom and Daisy (Rupert and Winifred's grandchildren) to be raised by their dear old great-aunt Amelia. (Amelia is currently living her best life as a single adult and headmistress of Copperdale Prep (a snooty residential facility for wealthy fail sons and fail daughters).
