Do you think that if Steven and Connie stayed fused that Connie would stop aging? Would Steven's powers of being able to change his age affect her through Stevonnie? Could Steven fuse with people, make Stevonnie younger, and effectively make the other person and himself younger once they unfuse? We haven't seen Stevonnie use Stevens shield, right? Maybe they can't use Steven's other powers. That would be weird, but would answer the age question.