The youngest shonen heroes in anime are also among the most common type of hero seen in these series. All the Ash Ketchums, Gon Freecs, and Kid Gokus are made to appeal to the shonen, or "young boy" demographic. Still, even taking the pandering to a certain demographic into consideration, some protagonists are absurdly young, to the point of stretching plausibility. Fortunately, most of these anime require a pretty hefty amount of suspension of disbelief as it is.

https://screenrant.com/youngest-ever-shonen-anime-protagonists-ranked/