I would love it if they eventually revealed that the reason behind the Richards' kids fluctuating ages was because it was being done deliberately as part of some plot by Dr. Doom, or an attempt by Reed to control their emerging powers, and that Franklin's latent abilities keep shifting reality so no one else can tell what's happening. Hell, make it both, and have Reed and Doom in a years-long tug of war over whether Franklin's hit puberty or not.