I think one's weight is a huge factor, along with experience. I'm 62 and I know people older than I am who can climb as well as many much younger cyclists. These are thin or even tiny people who have been climbing for years.
I'm 210 pounds, or so, and I can do lots of climbing when I feel good but I can't stay near guys who are 60 pounds lighter than I am.
Our local mountain rides are typically 100 feet of gain per mile. I'd like to do 63 miles with 6300 feet of climbing for my 63rd birthday this month but I might not be ready.