some scientists were trying to find a way to create eternal life. During testing they found it worked but, not in the way they wanted it to. Instead of keeping the subject at their current age it made them seven years old unless the subject was male in which case they also became female. The science team was afraid that this potion would cause the apocalypse so they tried to dispose of it. Unfortunately it leaked into the water supply and by morning everyone in Los Angelas had become a seven year old girl. These people had also become seven year old girls mentally as well so nobody was able to run Los Angelas. The little girls who used to be the scientists worked night and day to find a cure. All that they managed to do is extend the age range from just seven. Now whomever injested the potion that wasn't already affected by it became a girl in between three and ten.
Years past and much to the scientists frustration they wouldn't grow past ten so nobody took them seriously. As revenge they spread the potion into every country in the world.
Not everyone has ingested the potion yet. In fact there are many that haven't consumed the potion.