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It is best to age down with mods, but beware that you can still get some issues with sims behaving as the wrong age, especially if you delete, summon back, or save a sim at the wrong point (or edit with SimPE).
Make sure to not age down past toddler (can cause "forever infant" syndrome, where the sim will look like the age they started out as, act as an infant and not be possible to interact with).
Elder to adult can sometimes cause some issues with sims' hair (because of elders' grey hair), and wants/fears/aspirations can occasionally get a little bit messy too when aging down, but it should in most cases be fixable.