Instructions
Press "Cntl-Shift-C" to open the game's cheat prompt. A small window with a blinking cursor will appear at the top-left of the game's screen.
Type "testingcheatsenabled true" without the quotation marks and press the "Enter" key to enable cheat codes in the game.
Hold the "Shift" key and click on the Sim you want to edit. A list of options will appear around the Sim, most of which enable you to change characteristics of the Sim. For example, to change the age of the Sim, click the "Force Aging" option. A window will appear allowing you customize the age of the Sim.