Despite starting his legendary journey 23 years ago, Pokémon protagonist Ash Ketchum has remained a 10-year-old since 1997. This fact might just be credited to nonsensical cartoon logic, but Pokémon fans think there could be an in-universe explanation for Ash's agelessness.
Ash debuted in the anime's first episode, "Pokémon - I Choose You!," which first released in Japan on April 1, 1997. He has since continued on to be the protagonist of all seven Pokémon anime series, as well as nearly all the Pokémon movies. Ash has never officially appeared in any of the many mainline Pokémon video games, though Ash is a suggested name for the player character canonically known as Red in Pokémon Red, Yellow, Fire Red, and Leaf Green. While the events of Pokémon Yellow are based on the anime (Red receives a Pikachu based on Ash's), Ash himself was actually based on Red, as the Pokémon Red and Blue games released in Japan before the anime.
For the most part, each new Pokémon generation has featured a different 10-or-so-year-old character in its games, but Ash has been the anime's chosen hero for every generation. He's hopped between the world's various regions on a quest to be the very best, remaining 10 the entire time. In a 2003 Official Pokémon Website post (via Archive.org), a site editor stated simply that Ash "is ageless" because he's "an icon" and "a legend, like Mickey Mouse." This doesn't quite explain how time and ageing work in the Pokémon universe, however, so fans have come up with some other explanations.
Why Pokémon's Ash Doesn't Age Past 10
Many theories exist to explain Ash's age. One possibility comes from Amino user SomeNorthernGuy, who suggested Ash's body was frozen in time when Mewtwo turned him to stone in Pokémon: The First Movie, preventing it from aging but otherwise allowing Ash to function normally. A more well-known Pokémon fan theory suggests Ash fell into a coma after he crashed his bike in the series' first episode, and the rest of the events take place entirely in his head. But the most plausible explanation has to do with a very specific Pokémon.
At the end of the anime's first episode, Ash and Pikachu spot the Legendary Pokémon Ho-Oh flying overhead. This was a special moment for the series, since Ho-Oh's name and design hadn't yet been revealed, but it's also the source of the most logical cause of Ash's eternal childhood. According to the Pokémon Diamond, Pearl, and Platinum games' Pokédex entries for Ho-Oh, it's said that those who see it "are promised an eternal happiness." Since Ash presumably loves nothing more than adventuring with Pikachu on his Pokémon journey, the theory goes that Ash's version of eternal happiness is to continue that journey forever, and Ho-Oh's appearance granted him that wish. Like the coma theory, the Ho-Oh theory's reliance on events in the anime's first episode gives it extra weight, as it can potentially explain everything that followed.
Of course, at the time "Pokémon - I Choose You!" aired, it's possible Pokémon's creators hadn't even decided on that part of Ho-Oh's lore, especially since it wasn't worded exactly that way until Diamond and Pearl. But they also may not have decided Ash wouldn't age yet, so they would not have needed an explanation for that fact at the time. Perhaps The Pokémon Company included this part of Ho-Oh's Pokédex entry as a sort of hidden retcon to explain Ash in the lore. After all, it seems awfully convenient that the exact Pokémon who could offer a perfect explanation is the same one that shows up at the end of Pokémon's very first episode.