If you want to modify every aspect of a sim, including age, face, body type, personality traits, etc., you'll need to type in "cas.fulleditmode." It's one of my favorite cheats, since sometimes there are no teenagers in your neighborhood and your household's teens will need friends their age, so I just turn young adults back to teens. Another useful cheat is bb.move objects, allowing you to place objects on top of each other and move around easily. You can also resize objects by clicking on Shift and [ (smaller) or ] (bigger) while on build mode.