How to make my sim younger in the Sims 4?

You can't make them younger but in the age settings you can pause aging.. that may help your situation if your trying to prevent death. read more

You can turn off ageing in the options panel. This will automatically stop sims in your household (or world, depends on which checkbox you ticked) ageing. If your sim is already older than the life stage you want him to be at, then turn on the cheats console, type “testingcheats on”, click enter, then type “cas.fulleditmode”. read more

Aging is part of life itself and as obvious, it’s part of the Sims world as well. However, it’s always disappointing to watch your favorite sim getting old and eventually die from natural causes. read more