I had 60 year old student who basically untrainable. He was incompetent, and flying with him was nervewracking because he had no clue what he was doing or why. Ask for a 30 degree bank and you would get 60, wild stuff. It was a shame because he really wanted to fly but his eye-hand skills did not develop after 70 hours of trying to develop them. After about 10 hours with me (60 with others) I let him go and recommended that he quit. It was agonizing for both of us. We discussed some possible alternatives for him, such as model airplane flying and simulator flying. But for someone who is still being trained, I would not recommend using a sim unless they use it for instrument work or perhaps procedures that do not require fine aircraft control. And at some point if someone can't do basics like land an airplane without having a tail strike or leaving the runway inadvertently, they simply can't be a pilot. At that point they need to come to grips with it and find another hobby. It is your duty to discourage them at that point.