As it's a kid's show, any character is going to be interpreted from the perspective of children. You'd get a simplified version of what an adult character is - mostly experience, reliability, and maturity - and none of the things that make adult characters interesting.
For as much as he's part of the main cast, Kukui is a good example of this. We only ever see Kukui as a teacher and guardian for Ash, but we never see all the other responsibilities he has, the stresses of his work life, anything other than a romanticised depiction of married life, etc.
I don't think we'd be able to tell the difference between an adult and, say, a teenager like Brock or Cilan.
This is a good point. An adult character in a children's show is going to be interpreted from the perspective of children, the target audience. So, regardless of how interesting the adult character is, we would probably get a simplified version of an adult character.
Brock and Cilan, despite being only teenagers, more or less served the role of the adult in the group. Even Clemont, when he wasn't being browbeaten by his sister, served as the adult of the XY group. They were the mature ones, the knowledgeable ones. An adult main character would serve the same role, but might not be as relatable to the target audience due to the larger age group between him/her and the child characters.
Ash himself also might not bond as well with an adult character. He is well past the point where he needs a mentor; and he is less likely to form a brotherly relationship with an adult main character, due to the age difference. An adult main character would potentially change the dynamic of the main group.
I don't know guys. Having for once an adult character could work, if they handle him/her correctly. I always wonder, why nowadays every animated show that aims for kids must feature also a kid as the main focus. Back when I was a kid we had other animated kids shows that featured adult characters: Donald Duck, Scrooge McDuck, Baloo etc. I mean, if it worked back than, why shouldn't it now?
Why do most animated shows aimed toward children feature children as the main characters? Because the target audience is children, and people tend to relate best to characters who are around their own age and/or have similar life experience. It's for the same reason that many shounen anime feature main characters who are preteens or teenagers; that's the age of the target audience. Sure, years ago, children's shows might have featured adult characters, but if you think about it, those characters often had childish traits that were still relatable to young children.
