Tai chi pays close attention to how you are using your body. If you are feeling discomfort, it is possible to adjust what you are doing and solve the problem. People do suffer from injuries and illness as they age. And these limitations cannot be ignored. Much of the deterioration is the normal and acceptable outcome of growing older.
If you are over 40, you are unwise to ignore your body and force it to pretend that its 20 years old again. A 'no pain, no gain' attitude will only lead to suffering. Go easy on yourself. Do not put yourself under pressure or duress. What would be the point?