Ash Ketchum is the protagonist of the Pokemon anime, whose goal is to be the greatest Pokemon Master of all time. When the show began back in 1997, Ash started on his journey throughout the Pokemon world at the age of ten, and several years later, he is still ten-years-old. There have been many fan theories as to why he's perpetually ten. Some range from coma-induced dreams, while others look at evidence within the series to answer this commonly asked question of why this young trainer doesn`t age.
The Coma Theory
According to this Creepypasta-inspired theory, during the first episodes of the Pokemon anime, Ash is accidentally electrocuted by one of Pikachu's attacks, and he then slipped into a coma. The Coma Theory explains some of the basics from the series. It answers the question as to why Ash never physically ages, and it also explains why there are Nurse Joys and Officer Jennys throughout the region, and it also explains why children are sent off on journeys throughout the Pokemon world without adult supervision. It also explains that characters like Brock or Misty were actually figments of his coma-induced imagination, and it also states that Brock represented his sexuality, and because Ash never knew what sex was, this was the answer as to why Brock never became involved with any of the women he tried to woo. Misty's only in his dreams because she was based on a girl that Ash knew in real life prior to his accident, and he simply added her there. Giovanni was supposed to be Ash's father, and he's represented as villainous in his imagination because Giovanni had disowned him for his supposed sexuality, and Jessie and James were supposed to represent his purity since they were at that time the only Team Rocket members who wore white outfits rather than the typical black outfits. Ash's partner, Pikachu, was also supposed to represent Ash's humanity whenever he would get lost from him in an episode. The theory also notes that if Ash ever did wake up from his coma, he could most likely suffer brain damage from his accident, and that he'll have to live with the fact that none of his accomplishments in life actually happened, and that none of his friends were actually real.
In several versions of the Pokedex that revolves around this Pokemon, it is believed that the feathers of this Legendary bird gives the user eternal happiness. At the beginning of Ash's journey through the Kanto region, Ash catches a glimpse of this Legendary Pokemon, and it is believed that he never ages physically, because this is what he considers to be his eternal happiness.
Ash is DEAD Theory
In the Pokémon Episode "The Tower Of Terror" a chandelier fell on ash and pikachu. Haunter then pulled their souls out of their bodies, causing them to become ghosts. At the end of the episode Ash and pikachu returned to their bodies, making it possible that Ash and pikachu are ghosts possessing their old bodies.
The Time Lord Theory
This is a bit of a crossover but time lords age very slowly. They can regenerate to heal themselves or to change appearance (Explaining why Ash changes looks). So it would make perfect sense if Ash were a Time-Lord from the planet Galifrey.