This is a controversial study, apparently what they did to test for how exercise affected aging was they decided to control for "confounding" variables such as high blood pressure, obesity, etc. Except, these aren't confounding variables but mediating variables.

There was actually a youtube video about this very topic that I think is pretty good:

https://www.youtube.com/watch?v=Ic4nUxMYBRk

The takeaway is: If you are not training for marathons or the tour de france you are probably not overexercising to the point of causing negative health outcomes