It’s hard to develop people. In the first place, I think it’s a task that everybody can not do. You have to learn well before teaching. Sometimes it is necessary to make a tough judgment as to how far to work. So, regardless of age, I think it’s natural that some people can and cannot do the task.
Recommendation: Toku wa Ko narazu (by Motohiko Kimura, a Japanese book)
*The title means “Act morally,(and people will understand you in the long run)” which is written in “the Analects of Confucius.”