In some interview by some important guy, it was revealed the reason why Ash is still the main character because they need to keep Pikachu, the series mascot. Yet they still want new viewers to be able to relate to the 10 year old Ash the same way we did. This is why Ash will never age or have his character changed.
Which is a horrible idea for a show dedicated to a journey and personal growth. The fact that he forgets how to catch wild pokemon in the beginning of Unova proves that any character "development" Ash has means nill.