"The man should be older in thinking , He should be an innocent child in behavior & He should be young with respect to strength."
According to me, this statement shows optimal level of knowledge, ego/jealousy, strength with request to person's age. As we all know that as we are getting older our knowledge is increases since it is an integration of experiments & experience. I think it is quite clear that as child we are un-aware of social aspects of life, so our attitude, ego, jealousy, etc. are at lower optimal point, so it is beneficial in eliminates conflicts of life rather then I would say most of the problems. And we are having highest strength in young age.
So, the point here is to use such optimal level of described properties of man to achieve our ultimate goal of life or milestones of our life.