In The Sims 3, the adult stage is no longer the default age when entering Create a Sim, having been replaced in this role by a new Add an image to this gallery .
This action will change their age to young adult, and graduating will change it to adult, just like normal Sims. Also, PlantSims have only three of the seven life.
One thing I've noticed is that young adults can't be the child of an adult? October in The Sims 3 General Discussion it says) then you'll be able to remove the trait you got that you didn't want and add one you do want.