To see this glitch, you have to use testingcheatsenabled on, and you can only do it with an adopted Sim because you can only edit adopted Sims in create a Sim. After you adopt your Sim, age them up into a child. Right click them and then select "edit in Create a Sim", and then change your Sim from child to teen, then exit Create a Sim. This is what should happen:
Your Sim should stay looking like this for around a minute before he/she returns back to normal. :)