I was reading about the testingcheatsenabled true cheat "age transition" (Help & Cheats > Cheats > "Testing Cheats" section) and noticed in the paragraph that it states the only way to age *down* is through the reward "young again" or by using ambrosia. Well, I discovered a third way! With testingcheatsenabled true, when you shift click on a sim and choose "Edit in Create-A-Sim" you can actually choose their age group there! I did it with a wild horse I adopted and wanted to breed, but she was an elder. Not anymore!
(I have the Sims 3 and Sims 3 Pets on PC, for reference.)