Yes, we sooo need a life state between Adult and Elder. I am now playing with aging off so I age my Sims, and also kill them, manually. My rule is that once a child becomes a teen and their biological parents are YA, I usually age those parents to Adult. If the parents are already Adults I don't usually age them until said kids are YAs. Because in my game the Adult stage is longer (from 30 years old-to around 60-65ish? even up to 70) than the YA stage (18-30 years old). The same when the first kids become YAs. Then the parents become Elders. I do sometimes keep their natural hair color instead of the grey/white they get when they become elders though. I find that the transition isn't so jarring.
In my game, I assume that the adult stage lasts until the early 60s, mostly because we don't have a middle-age life state. In my main household, the parents are adults, their two oldest daughters are young adults and the other three kids are teenagers, one of them has just started college. I usually age up teenagers to young adults once they have graduated and start their first job, simply because they start college at around seventeen, so I'm assuming they're maybe twenty-one when they graduate, depending on how long it takes them.
