I don't think it's wrong for non ados to play ados roles, but reasons like the op is why I prefer it to go to ados actors. I forgot which poster I was going back and fourth with on this but you could be as classically train and have all the facial and timing nuance in the world but if the accent isn't accurate, it's a huge distraction for the person you're trying to play.
and also, alot of these Brits who are classically trained, their acting don't always translate to the big screen. I'm not saying that's the case with Daniel Kaluuya, but when you're given a person like Fred Hampton to play, whose way of talking was really charismatic and had a slight countryish chicago accent, it's gonna be even harder for a classically trained Brit, who were only familiar with the white southern american accent, but not black. He can train for it but why not give that role to a classically train black american then(and don't tell me there's no classically trained black americans)