Just discovered this thread. I'm 64, started skiing at 36. My skiing is constantly improving, not slowing down at all. I ski 60 or more days a year, this year I'm aiming for 90 times, already at 41. Yesterday was my 1,000th ski day of my life!
The only difference is that I really don't want to fall, so I don't push my limits as much as when I was younger, so yes my improvement may be a bit slower, as pushing your limits can accelerate improvement. That said I'd rather ski more days than get hurt and miss time on skis.
I'm skiing steep and icy trails better than I could when I was younger. I'm skiing ungroomed terrain better than I did when I was younger. The better one gets, the less physical effort it takes.
I have a fellow instructor in his mid 70's who still skis athletically. I guess at some point in my late 70's or so I will have to limit what I do, so maybe I only have 10-15 more years of athletic skiing, but hey, skiing easier trails in my 80's will be just fine with me!
There are folks in their 90's that I know who are still skiing, and skiing fast.