Shift-click the trash can and select Reset Age Duration.
This will (temporarily) delete your Sim, enabling you to restore them in a teenage body. Then, shift-click each Sim, click Set Age, and then click Adult. They won’t ever be 'true' teens because they won’t need to go to school, but they can get jobs, 'WooHoo', have babies, and even drink the Elixir of Life to stay young forever. Don’t include the quotation marks or forget any spaces, which will screw up the code.
