Good morning, Loong. I'm sorry to hear that you have such health problems in your knees and shoulders. My joint problems are in my feet and left hip, and I know the pain of bone-on-bone arthritic-type impairment. I do find that Tai Chi is gentle and accessible to me. After spending 30 to 60 minutes in Tai Chi, my joints feel better than when I began (unlike most other types of exercise, which leave me more sore for that day and the next).
Many of the moves in Tai Chi involve slow movement of the legs in a bent knee position. The exerciser determines how deeply to bend the knees. These moves may be too painful for you with your knee issues: You would have to determine if you could adapt the movements in a way that is healthful and comfortable to you, with consultation with your physician and/or physical therapist.
The arm movements are slow, gentle and graceful, using isometrics to gently tone the arms and shoulders. Again, you could adapt them by limiting the range of motion, but should consult with your physician and/or physical therapist on this.
Many local libraries have exercise DVDs available for checkout. Mine has several Tai Chi programs available. Perhaps you could check out a couple to view to determine if this may be something that you would benefit from and if it would be safe and helpful to you. Alternatively, you would be likely to find video samples of Tai Chi on the internet. That way, you would not incur the cost of buying a DVD without some determination of if Tai Chi would be good for you.
I understand your frustration and sadness over having to give up athletic endeavors. I was never really an athlete, but I did study dance for many years, and enjoyed performing as well. I miss that terribly. Learning Tai Chi has relieved some of my sadness in losing dance as I learn and increase my abilities to perform the graceful movements of Tai Chi.
I wish you much health and happiness! If you get the chance, let me know how your assessment of Tai Chi goes for you, and if you are able to develop a practice.
Jori