It's not exactly news that exercise might delay aging at the cellular level... but this study strongly suggests that the effect is strongest in middle-aged people between 40 and 65. The types of exercise in question were interestingly different from most of these studies too: equal weight was given to weight training, moderate cardio, vigorous cardio, and commuting by foot or bike!

I started with weights and moderate cardio in my 40s after a hiatus and it definitely has helped me feel good. Even though when I first started getting back into it it was hard.