I think Dexter and the Powerpuff Girls were aged up for a specific reason.
In the show, Dexter's Laboratory is a secret location, his parents are completely unaware of his genius and technology. I think he was aged up so that he can be the owner of a multibillion dollar corporation, "DexLabs", that's known world wide with technology that has revolutionized the world. So now, in this alien invasion crossover, Dexter, as the first Cartoon Cartoon, can be the main character and lead the battle against Fuse and everything. The existence of DexLabs gives Dexter the ability to be the main character and participate in this crossover scenario so actively.
With the Powerpuff girls, in the show, they were just too young. They were kindergartners. They were aged up to fit in better with the world, and the age range of the other kids being in elementary, middle, or high school. And due to the nature of their characters and show and stuff, there's no drastic change in status quo by aging them up aside from their daily civilian level lives being updated.
With the Kids Next Door, Billy and Mandy, Foster's Home, and Ed Edd 'n Eddy, there's no reason nor benefit to aging them up. They are already in a good age range, 8-12, and they are generally supposed to be kids. In FusionFall, they can just have the canon of their show transplanted into FusionFall with no alterations in timeline placement. Plus, these specific shows take place in the same canon.
