Use the Insimenator or the Temporal Adjuster object which is a part of the Insimenator to change her to any age at any time.
http://www.insimenator.net/forumdisplay.php?f=38One caveat, though - if reverting a Sim to a baby, best to also make that reverted Sim then unselectable, as having babies being selectable Sims and then Saving the game can possibly cause game problems in some instances.