My husband and I are currently hiking the CDT - he is 66. He was told before we left that he needs knee replacement surgery - so pain has been a big part of this hike. However he is very determined to keep hiking, so we do. He did his first long hike (AT) at 54, second (CDT) at 60, then third (PCT) at 61. There were problems with knees on the AT, but not on the PCT because it is graded so gently. He has had foot problems on all trails (blisters mostly, but also nerve issues and a broken foot on the PCT), and back issues (sciatica) but again, if the will is there, you can continue.
Biggest thing with older hikers is to keep the pack weight as light as possible (not so easy on the CDT with long food carries, heavy water and cold temps), and to remember that injuries take longer to heal as you get older, so take it easy to begin with so you have fewer injuries. We know 60 plus hikers who regularly do 30 miles a day but we generally do 16-20. On the AT you can easily complete the trail with an average of 12 mpd. So go out and enjoy.
Spirit Walker