After one movie Asher Angel is already 17, so it's not going to take that many movie.
How old is his character, though? Most teenagers in film and TV are played by actors in their 20s anyway.
Isn't the whole idea behind the character that he is a kid who turns into an adult superhero?
In the original comics, yes, but how that's been interpreted has varied -- originally it was basically as if Captain Marvel was a separate, adult persona who switched places with Billy, and it's only been in recent decades that it's been reinterpreted as Billy's childlike personality in an adult body.
Anyway, the operative word there is "behind," not "ahead." Just because a movie series starts from a certain foundation doesn't mean it's forbidden to grow beyond it and become its own distinct thing. There's no precedent in the comics for Thor joining the Guardians of the Galaxy, or for Wolverine being the caretaker for Professor X in his old age.