This challenge is inspired by, but not the same as, the 100 Baby Challenge!!
Abstract Goal: Your goal is to keep a sim in the Young Adult stage as long as possible using the gameplay mechanic which stops sims from aging during pregnancy. This is not a mod or a cheat - it's just how the game works - so anyone can do this!
Concrete Goal: For your ONE pregnant sim to have had 100 babies with 100 different sims during the Young Adult life phase only, before aging up into Adult. This goal is accomplishable using regular lifespan.
The Eternal Youth Challenge is different than the 100 Baby Challenge because you do not have to keep the babies in your active household for a set amount of time. You simply have to have each baby, and you are welcome to move out babies/children at any time, as long as you keep at least four kids of any age in your household.
Once you start having babies, implement a 5-sim household minimum. You must keep at least 5 sims in the household (the pregnant young adult + four children) at any given time. However, these children can be any age. For example, you could choose to keep babies 1-4 as they grow older and then move out baby 5 as a newborn or infant; or keep babies 2, 5, 7, and 8 while moving the others out.*
Though you are welcome to use a nanny, maid, etc., you are not allowed to have any other young adults, adults, or elders living in the household with you. For this reason, you may choose to keep some of your babies until they age into teens so they can help take care of the babies. Once your baby ages up into young adult, though, you must move them out.
How it's similar to 100 Baby:
When it comes to aging up your babies EARLY, the 100 baby rules apply. Personally, I just let the children live out the whole life-stage and don't worry about trying to age them up faster. But as a reminder, those 100 Baby milestones are:
Infant: at least 3 milestones
Toddler: reach level 3 in all toddler skills
Child: Earn an A in school
Teen: Earn an A in school
Remember, though! You can move out your babies at any point in their life, so it's totally fine if you just let them age naturally, and/or don't think about the 100 Baby milestones at all.
If your computer performance can handle it, this challenge is the most fun when you keep all the babies in the world instead of deleting them from existence.
A recommendation: Though not a rule, I recommend that you create one extra household to begin with (you can choose to move them into a lot, or not, up to you!) where you will move the babies when you need more room in the active household. For this, you can create a relative of your pregnant sim (for example, their parent or their sibling), or any other adult guardian of your choice, who will "care for" the babies. This is important because you won't be able to move a baby or child into their own household without an adult present.
Once you have more than seven babies, you'll have to create a second household for the next set of babies. I recommend simply using one of the older babies who will has reached teen or older by then.
Tips for success - NOT Required
Give your pregnant sim the Serial Romantic aspiration, and complete it as you begin this challenge. The rewards are helpful! Although I personally like playing this challenge with no cheats, I do use the reward point items (like the Moodlet Solver potion) when needed.
Once your sim is pregnant, start lining up the next donor! (Gender is unimportant - this challenge only requires the "get others pregnant" option for "donors," and the "get pregnant" option for your pregnant sim.) Even better? Have your sim invite the next donor over when they go into labor. Then, immediately Try For Baby after giving birth. Getting pregnant on the same day that you gave birth is the key to winning this challenge.
Name your babies after their donors (for example, Bob Pancakes's baby could be Bob, Bobby, Bobbie, etc.) to help you keep track of who you've had babies with.
AND/OR, keep a list or spreadsheet of each baby and their donor. This will help you keep count and avoid accidentally duplicating a donor.
Too hard?
Remove the 5-sim household minimum (and/or)
Permit ONE other adult, preferably one of the oldest children, in addition to the pregnant sim (and/or)
Give yourself both the Young Adult and Adult phases to have all 100 babies
Plan stayovers with adults, such as donors or friends. You can have these adults help out around the house, and/or use this feature to ensure that the next donor is ready to woohoo, even if you go into labor in the middle of the night
Too easy?
Require yourself to have a baby with all of the townies who can "get others pregnant" and/or everyone on a specific list of favorite townies (and/or)
Start your game with §0 (and/or)
Require that your pregnant sim have a full-time job (and/or)
Require your sim to not earn money through art, crafts, writing, music, collectibles, gardening, etc. (and/or)
Require that your sim earn money only through dating and/or the "Ask For Loan" interaction. That is, make your money by selling the flower or VIP Bucket reward items after a successful date. (and/or)
If that isn't hard enough on its own, limit yourself to ONE date or loan per NPC (and/or)
*Implement a 5, 6, or 7 sim minimum household size! (Pregnant sim + 5 or 6 children.) You can't require an 8-sim maximum because this will inhibit your sim from getting pregnant again.
ENJOY!
If you play this challenge please tell me how it goes!!