I've not done a personal tour since major surgery three years ago, but still bike to school (at 64) and locally. I've been leading group tours (and riding my own) for over 25 years and now drive the sag van all the time instead of sharing it with others. Perhaps my favorite part is scouting new routes for my riders, and I came here with an Adirondacks question for this August's tour, but no answers yet.
Out of curiosity, perhaps other than some age-related aches and pains, just what makes cycling different at any random/particular age - such as 50? A lot of the riders on my tours are in their 50's, 60's or even 70's, and one year on my Memorial Day Dash to Montreal trip, the two strongest riders (out of 24) were 75 year-old guys.