Exercise can keep you younger at the cellular level.
This is about your telomeres, the " tiny caps on the ends of your DNA". They protect chromosomes from deterioration and naturally shorten with age. The more you lose, the older you are. Smoking and obesity were already known to shorten them. This study shows that exercise protects them. https://academic.oup.com/aje/article/doi/10.1093/aje/kww196/2915786/Associations-of-Accelerometer-Measured-and-Self
Although the study is about elderly women and their ten hours of sedentary life, and how exercise can negate aspects of that, it is worth reflecting on current averages for younger people. We average 3.5 hours of TV per day. Many jobs have us at desks for longer than 7 hours. We beat the old for sedentary. https://www.statista.com/statistics/487130/average-tv-viewing-time-per-day-uk/
Men don't escape - the average rate of telomere decline is greater in men than in women. However, on the plus side, there are studies that shows how lifestyle changes and exercise can actually lengthen telomeres. https://en.wikipedia.org/wiki/Telomere
The 7th January 2017 edition on New Scientist had an article called “Time to stop getting old”. It discusses “the race to develop anti-aging treatments” and describes several pathways under investigation. There is the “young blood plasma theory” which is about blood transfusions from the young to old. There are attempts to protect the telomeres. Mice gene therapy, 40% of mice lived longer, but the therapy is not ready for humans. But some have tried similar therapy, plus gene therapy to prevent muscle loss. Then there is targeting senescent cells to kill them off. And heavy fats – the heavy isotope of hydrogen is the interesting part. Near the end, after talk of supplements and tests, a chap from Stanford University in California says “But the number one thing that we know can protect your telomeres is exercise.”
I emphasise that: “…the number one thing that we know can protect your telomeres is exercise.”