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Even though I know it's just cartoon logic so there's no need to try to explain it, I like the theory that Ash meeting Ho-Oh at the beginning of his journey somehow made everyone in the pokémon world immortal, while also making them not realize what happened and keep living normally.
People clearly aged before Ash left Pallet, but the current situation can't be a groundhog day thing since there's continuity to the events in the show.
If Ho-Oh's feather was enough to split the timeline into two - the anime and the current movies-, then why couldn't it do other timey-wimey stuff?