In reply to Chipmunk:
I'm 72 and know what you mean.
Descents are definitely becoming more of an issue, but I just go slower.
I've seen various doctors/professionals over the last few years concerning aches and pains in both knee and foot joints, and definitely find poles help for long descents. Exercises recommended by health professionals definitely help as it seems that keeping your muscles well toned is increasingly important.
Good fitting boots are essential as they give you lot more confidence for accurate foot placement. I have also started using shaped in soles for improved arch support which made a big difference. I find wearing varifocals awkward in descent because they make you bend your head down more to focus.
I have just accepted that I am slowing up generally but so what? My stamina is good and if a route which has a recommended time of 8 hours in the guide book now takes me 9 hours, so what? I still do it and arrive feeling relaxed and not too knackered.
The only situation where a brisk pace is important is in Alpine mountaineering or Scottish Winter Mountaineering, so I am tending to go for outings where timing is not so critical.
Good luck and enjoy another decade of mountaineering at whatever level suites you!