Oh, I'm not saying that Jane would have been able to maintain all the skills she developed in childhood or to be even near Chita's level. Not many are.
What I meant to convey is that while age is certainly one factor, it needn't be the only factor. People who continue to do what they did during their early years can maintain many of their skills well into their later years. Judging from Jane's body, I would suspect she has done that to the extent her injuries would allow. It wouldn't surprise me at all if she had a ballet barre in her home.