In reply to cander:
There was a recent thread on this subject started by someone called Roger Wilco. I'm nearly 75 and I've definitely noticed that my legs don't have the old spring and shock absorption in them that they used to, and my eye sight is not as sharp either, even with glasses. I used to bound down hills jumping from rock to rock with ease. Going downhill is definitely the worst and I am slow now, anxiously peering ahead trying to assess each foot placement. Walking poles help but they are a bind to carry when scrambling.
I also get tired after a long day in the hills, and if I do several in a row it becomes really knackering. But build in more rest days and the old body quickly recovers.
I've only noticed this over the last 2 or 3 years, so in my 50s and 60s it was no problem.
I also mentioned an increasing difficulty in hearing people coming down faster from behind and overtaking, particularly mountain bikers, so please please use your bell well back when coming up behind someone, and if they don't react slow right down. Shouting isn't always enough, a bell is much easier to hear, and if they haven't heard you don't startle them. The same applies to Fell Runners, if the person you are overtaking doesn't react to a well back shout, slow right down and overtake with care - if possible go a bit off piste