I think the elderly is not suitable for learning martial arts, because martial arts is a strenuous exercise program, the physical quality of the requirements of the very high, and the elderly body functions in all aspects of the degeneration of osteoporosis is also very common, so the elderly in the practice of martial arts will be very easy to cause bone fracture, so the elderly is not suitable for practicing martial arts.
See there are a lot of older people in the park to play Tai Chi, in fact, Tai Chi is also a dangerous sport for the elderly, because playing Tai Chi on the knee is very big damage.
So people with original knee injuries are not suitable for practicing Tai Chi. Even if there is no problem with your knees, you should practice under the guidance of someone who knows what they are doing.