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I think agin him 1 year per gen would have been the best approach
The way he acts from gen 1-4 is acceptable for someone 10 to 14 sure it would make him look even more like a retard if he was 15 in gen 5 but having serena liking him in gen 6 seem more natural if both of them were 16 instead of de aging serena, im gen 7 all you have to do is change the school a bit and keep the trial captains the same age as the games gen 7 is special because he wins right before becoming an adult also he got to learn a lot from living with kukui, and by now he would be 18 working as a helper for sakuragi, meeting old friends and trying to become number 1 worldwide would feel natural for a young adult ash
Maybe just maybe you could even keep goh 10 and have adult ash teaching him things just the way he does already i remember at the start of journeys some scenes with goh that mimic gen 1 ash those could have had more of an impact