Are you asking me about Baryshnikov? He's 59 years old, and has had almost a dozen surgeries, including one on his knee just a couple of years ago. He isn't at his peak physical powers anymore, and that means he can't jump as high or as often. Speed and purity of technique can be affected too, depending on what you're trying to do.
Ballet's a very particular and demanding physical discipline, on top of all the artistic stuff you have to add, and generally can be done best only by those in their physical primes. I've seen Baryshnikov recently in his modern dance performances, and can appreciate the man's charisma and artistry, but I wish I had seen him in his prime dancing an art form in a way many consider to be among the best ever.