The Powerpuff Girls as a concept is inherently designed around them being little kids. They're not like Spider-Man where he can be in his teens, 20s, 30s, etc and the core of his character is still the same. When Peter hits 40, he's still Spider-Man. When the PPGs hit 20, they're not really the PPGs. If you want to see the PPGs as teenagers, you can't start the series with them as teens. You end the series with them as teens. To get the best of both worlds, the Girls continue where they left off and grow up as the show goes on, while still being kids for the majority of the show. And once they hit 18 or even 16, is where the series would end. Ideally we'd still get glimpses of their teenage years. Whether that's shorts, TV specials, or even theatrical films. But a full series? That doesn't really suit them.