Well, i'm a bit of a wrinkly myself, but am very lucky with most of my joints. My main points would be
1)Warm up gently for 5 minutes at least before dancing to establish good blood flow. This is much more important than stretching. Stretching should be done after exercise
2)Keep exercising. I read recently that when jogging with arthritis, as long as you can keep proper 'form' (i.e. don't start favouring one side) the benefit of the exercise outweighs any slight damage being done.
3) Go to the gym to do general strength work, and keep flexible by doing flexibility work. What i really mean i guess is keep in good shape outside of the dancing.
4) Glucosamine sulphate has been shown to improve joint spaces in the knee. DEfinitely take 1000mg per day .It often comes with chondroitin
5) recent research has shown that for older people, a protein supplement taken within half an hour of exercise will speed up repair of minor tissue damage. Get some protein bars from the health shop.
6) Remember that the knee joint is essentially held together by your quad muscles and it's essential to keep those strong to prevent excessive joint movement. It is possible to exercise these well without bending the knee, and so avoiding any damage.
7) you'd service your car regularly, so do the same with the body. Don't neglect anything.
This doesn't help anyone, but there's a lot of heredity involved in these things. Also, it has been shown that those who have exercised all their life (well, nearly all their life, but you know what i mean) may have more joint damage, but cope with it much better than others.