>>28110220>>28110247I can answer that for you.
Originally, the idea was that the Pokémon journey was a "coming-of-age" adventure, that signifies maturity and responsibility. One of the original anime writers said that it's actually considered the start of adult life in the Pokémon universe. (Yes, you read that right. All the PokéGirls are totally legal. Based pervert Japs.)
Well Black/White were back when Game Freak decided a more mature take was what they needed to keep older fans happy. They doubled down on a stronger story, and older protagonists. (Hilda/Hilbert were originally even going to be slightly older, at 16, as reflected in the Manga)
I believe, and this part is just my speculation, that the reason they are going back to younger protagonists, is because the fact that there is no longer a "Chibi" overworld means that having mid-teen protagonists might be off-putting to children, the main audience of the games. So to compensate, they're going back to smaller, younger characters.
As for Ash, it's semi-common knowledge that Ash wasn't always going to be 10 year old. It was around the end of Gen 2 where they made the decision to basically "soft reboot", and bring in May. They basically decided to focus the anime on being an ad campaign for the current games, rather than an actual developing series. This is why Ash never aged. He has to stay the same age as the kids they are marketing to. This led to the writing staff quitting, the anime becoming very stale, and even the entire dub team being overhauled. Voice Actor Eric Stuart said himself that the reason he quit was that he didn't want to stay involved when the show became a glorified advertisement.
