Aging down a SIM

I go by Lilith131367 and am from Ohio,  I have a question: one of my SIMS accidentally blew the birthday cake candles and aged up to young adult, thereby skipping his teens and I want to age him back down to teen. I have tried the testingcheats true cas.fulleditmode shift and click on SIM click on CAS and make him younger but it says that pre-existing SIMS cannot be modified. How do I age him to teenager?