Thank you so much for the replies. It's honestly a shame how The Sims 4 doesn't have a comprehensive, Minecraft style wiki that explain how everything in the game works in a full detail. As for aging, this is everything I know from my experience and testing.
Every single way to reset sims' age to the beginning of the current lifespan I know of:
Base game:
- Potion of Youth
- Ambrosia (requires a Potion of Youth to be made)
- Cowplant's essence of life
Get to Work:
Seasons:
- Orchid scented flower arrangement
Romantic Garden:
- Overwhelmingly positive outcome when wishing for youth from the wishing well
Realm of Magic:
Paranormal:
Nifty Knitting:
- Giving some gift to the grim reaper. Forgot what that was since I don't own the stuff pack
Strangerville:
- Requesting life extension from the mother plant
Add more if you know
As for the bonus days in the elder life stage, here's how this works:
- The active trait (depends on the lifespan)
- The long lived trait from completing the bodybuilder aspiration (depends on the lifespan)
- Cowplant essence (10 days regardless of the lifespan)
Positive outcome from wishing youthful vigor (RG) (5 days regardless of the lifespan) - Passing "no regrets" milestone (L&D) (don't know how many since I don't own L&D)
This is how many days the active trait adds in each lifespan:
2 on short, 4 on normal, 15 on long
The long lived trait:
8 on short, 16 on normal, 66 on long
As soon as you change the lifespan, so do the extra days from these traits
Extra days obtained from the wishing well and the cowplant essence are counted as bonus days and stack. This means that if your sim drank the cowplant essence twice and got the positive outcome from the wishing well once, they'd have 25 bonus days. You can basically make a sim that lives thousands of days like that.
All the extra days obtained stack. If you have the active and the long lived traits, you get the bonus for both of these + days obtained by the cowplant essence and the wishing well.
Wiki says that "no regrets" milestone also gives additional days but I can't explore that as I don't have the expansion pack.
I read somewhere that orchid scented flower arrangement may also give additional days, but I can't tell how many as I also don't have seasons
From my testing, sims with the cheerful trait seem to live longer but I cannot confirm that as MCCC does not list the days obtained that way
Important note: on short lifespan there's a bug where the bonus days from essence and the wishing well are bugged. Meaning if you have 10 bonus days, you actually get 20. EA, it would be nice if you fixed that.
So here's some starting information to for you to work with if you ever decide to dig deeper into the game's aging mechanics.