Were you still trying to teach them that at 24 years old?But you don’t know that he is dumb, that is the problem. Currently he is a talented but inconsistent footballer. I think you are labeling him with a tag without there really being much evidence to support that tag. I don’t think Barkley is the savior of English football, nor do I think he would get in our current first team if all are fit (it is the same story at Chelsea too). However Barkley does have good attributes and furthermore they are the ones that cannot really be coached (power, skill, ability, confidence). The attributes that some perceive him lacking such as positional sense, when to release the ball, when to tackle/not, which runs to track, etc) are the attributes that are easily developed. When I was involved in youth coaching the talented youngsters without those other qualities were the ones you wanted as it was so easy to improve their game substantially.
If so, wouldn't that tell you something about their ability to comprehend simple instructions?