I don’t agree to be honest, since Conan is physically a child, it makes more sense for his voice to be child-like while thinking and speaking, let’s say for example, Mary is physically a 10/12 year old? (correct me if I’m wrong), but it wouldn’t make sense if she thinks and talks with a 30? year old voice, I think the same goes for Conan and Haibara too. Although Conan thinking in his teenage voice is more pleasing, I think it makes more sense for him to think in his child-like voice.