Macnyc--what you can't see in that video--and what the Bennett choreography was all about--is the way the ghosts came out of the darkness and gloom upstage. The video is lit for the awards show, not for the production.
The older women start out trying very hard to remember the routine, as the dance routine keeps getting more and more difficult. But Bennett would never let them play forgetting the steps or doing them badly. He loved to stage things like that with a very unique artfulness.
Then the tap section starts. At that point, the ghosts appear--facing upstage--as if the women are experiencing their memories being refreshed, the way dancers have what they call "muscle memory."
The lighting at this point became almost surreal, and the ghosts faced forward and began to mingle with the older women--the past and the present coming together. By the end, it doesn't matter that the younger women are out-dancing the older women. They are all doing it full-out, 100% to the best of their ability at their age.
So the song was not about how you can't dance anymore when you get old. It was one of the two moments in the show in which remembering the past was NOT destructive. It was rejuvenating. (The other was the crescendo of "I'm Still Here.")
Here's a bad-quality video of the number from the original production. Watch how the ghosts appear:
http://youtu.be/mgvoAr0_gt4