1A. Start a challenge with an existing old Sim that has died of old age (preferably in premade households, as those Sims have a lineage of relatives that were/are in their lifes), or (If you have Supernatural installed), create a ghost.
1B. Alternative way, you create a mortal Sim and have him/her attained necessary requirements/circumstance/opportunity to resurrect another deceased old Sim.
2. Once you have that Sim ready, play that ecoplasm Sim until they have remaining years to “cross the other side / nether”. HOWEVER, you must ressurect them to flesh before Grim Reaper can take them.
3. Then play with that mortal pensionar Sim until they reach their final days before dying of natural causes.
4. Once they about to go to a better place, use cheats or mods to deage them.
5. Repeat this with every lifestage, so instead of reaching their birthday, they jump back in the previous life stage they were followed by.
6. If this Sim has relatives and spouses of theirs, do the same with them. Sims that revert back to baby and reach their end of lifespan, if possible remove them from the household and make their conceiver back pregnant (cheat or try for baby) and subtract one day before they can give birth (If you want, you can try for baby before babies detransition and use woohoo before pregnancy to allude reversal of life)
7. If you want to make it challenging, you can reenact all the periods of life in reversed manner (retire, then climb career ladder, pregnate and unpregnate, do well in high and elementary school, relearn walking/talking/potty, babify and back into whom).
8. In addition, once a Sim goes through the whole life cycle and then ceases to yet exist, you can remove them with nrass from the family tree.
9. The challenge ends once you ended the legacy by reverting back to the founder/creator/heir of the generational family.


Hope you end this challenge