User blog comment:Psychopulse/Phineas and Ferb continues to expand, and a possible spin off involving the Fireside Girls.../@comment-406459-20100623022741

I doubt there would ever be a spin-off about older versions of the characters. For one thing, shows intended for children usually have child protagonists so that the audience can relate to them, and mainly because I don't really think there would be any humor potential in it.