Why do some anime characters never grow old? Ash Ketchum from Pokemon



By never growing old, I mean the anime has been airing for a long time, but the character never grows old. I think the most popular example for this kind of character is Ash Ketchum from Pokemon and Nobita and his friends from Doraemon. (Yes, I do realize that both anime are for kids - I don't remember any other example that the character never grows up for a long time). However, most characters from long-running anime that I know of always grow up, like those from Dragonball, One Piece, Naruto, Fairy Tail, and Shin-chan. (The last one I mentioned has similar traits to Pokemon and Doraemon, although I'm not sure Shin-chan is for kids). Any reason behind this?
The creator of Pokemon wanted children watching the show to be able to relate to the main protagonist, Ash/Satoshi (personally I believe the protagonist is Pikachu).
Keeping the character around the age of ten is a good way to get kids that will be this age (ten) soon to watch the show.