I think the Fountain of Youth from Hidden Springs (it's a Store town) does something similar, but haven't messed with it since there are other ways that get me to the same result. Drink it and it will put your Sim back at Day 1 of being a Young Adult without having to go through the Create-a-Sim stuff. If you have Supernatural installed, you could also visit the Elixir shop and purchase a Fountain of Youth elixir. It does the same thing (click the sim -> NRaas -> Master Controller -> Advanced -> Edit in CAS), but MC's CAS is less prone to corrupting your game than the developer console version.

However, if a ghost Sim drinks it, they’ll be revived, returning to the world of the living. If a living Sim drinks vitality nectar, their age will reverse, making them younger than before. I prefer to use the Master Controller mod with the Cheats add-on. As you play The Sims 4, you’ll naturally see your own Sims and those in other households pass away for various reasons. Save before doing it and make a new save immediately after doing it (Save As.). Change it to Young Adult or Adult and accept the change to return to the game. On the Basics tab, you'll see that Elder is highlighted for the age. Turn on cheats, shift-click your Sim and select 'Edit in CAS'. Sadly, I realised too late there was an option to turn it off and I already keep getting. Im pretty sure chicks have the shortest age up time of all the animals so unless one of them is close to death the chicks should be able to age up with no casualties. Originally posted by has the right procedure. If shift clicking doesnt work just turn aging on for a lil bit.