Is there a way to actually age down a sim in SimPE? I tried to just change the life stage to adult (from elder), but it left them with the elderly body.

SimPE changes the character data, but to actually change the physical appearance you need to run the game and get the Sim to a mirror for the "Change Appearance" option (I like to change hair color and then change it back, just to make sure it "takes").

In general, changing Sim ages is best done by the various in-game mods (Sim Blender, Sim Manipulator, Sim Modder, et al).

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.