I'm 51 and notice high impact at times bothers my knees (for some reason high tuck jumps especially, don't know why...), and in general, I really need take lighter days every now and then, but specific issues seem to come and go as others here described too. I'm still pretty much in the best shape of my life, never thought I could actually feel so comfortoble and healthy in my body, owing a huge part to Cathe!!
For me the biggest thing getting older has been to just get a lot more sensitive to what my body says. I think when I was younger, if I had decided to do an interval or other hard workout one day, I would do it no matter how badly I slept or what my body was trying to tell me.
I workout in the mornings, and usually take a few minutes with a cup of coffee to really see what kind of workout day it is... I think I get so much more enjoyment (and probably results?), when I really concentrate and think of the muscles when i work them, it give a great angle to lighter workouts too. So for me, that has really been the best part of working out at 50, no stopping in sight yet... ;/) Today was one of the Hardcore Extreme workouts (forget the name, but the one with 2 x Imax3 blasts + High Step Challenge weight segments x 3... and it felt so good...
). So tomorrow probably a lighter metabolic wo oa. Wish you good workouts - listening to your body!