This isnt a criticism cause i kind of understand your concern but i think its just his general moe look to him due to the artstyle. Categorizing boku no pico and him under the same umbrella feels like an overgeneralization cause there are a ton of younger looking men (in fandoms like genshin ie aether, xiao). Plus there are a lot of ukes in this site (cause size diff kink is common) and it just feels strange to point it out in this situation.

I really want to like this but I can't with the demon king looking like a 10yo boy like as bad as boku no piko. People defending it are secret pedos because wtf you mean "it's okay that he's physically 10 years old because he's actually x years" like some incel reddit mod defending his love of a 3kyr old dragon girl who appears to be 10.