We know the Pokémon anime is celebrating its 20th Anniversary. On episode 1 of Season 1, he was a 10-year-old when he received Pikachu. Shouldn't he be 30 by now? Is he supposed to be 10-years-old (11 in Alola) forever?
There are many theories as to why Ash is still so young, and also ones as to what age he would have if he had aged over the years. The theory I personally believe is that when Ash saw Ho-Oh in the first episode of the anime, it granted him eternal happiness, as that is what Ho-oh is stated to do according to the Pokedex. I believe for Ash, eternal happiness is getting to be young and travel with Pokemon forever. But of course there could also be no reason for it, and he is just kept at age 10 forever so they can keep a specific target audience.