Is 75 minutes of exercise per week already enough?
75 minutes of vigorous exercise per week already delivers almost the maximum benefit for your lifespan. On top of that, make sure you do not sit for long uninterrupted periods during the day, as that is a separate risk.
Yes, 75 minutes of vigorous exercise per week (or the equivalent: 150 minutes of moderate activity, such as brisk walking) is already enough to capture a large share of the benefit. People who meet this minimum have a mortality risk that is roughly 31% lower than completely inactive people. That is already close to the maximum benefit that exercise provides.
Doing more still helps a little: people who exercise two to five times the minimum end up with a roughly 37 to 39% lower mortality risk. That is additional gain, but modest compared with the jump from doing nothing to meeting the minimum. The ceiling sits somewhere around three to five times the minimum guideline. Exercising extremely large amounts, ten times the guideline or more, does not appear to be harmful for mortality risk, but it provides no extra benefit either.
The biggest gain comes from the first step. Even somewhat less than the minimum, so fewer than 75 minutes of vigorous activity per week, is already associated with a 20% lower mortality risk compared with people who do nothing at all. And light activity such as gentle walking also counts: the step from being completely inactive to being a little active is by far the most important one.
How you accumulate those minutes does not matter for mortality risk. Short bouts of a few minutes spread throughout the day produce the same result as continuous blocks. Total volume is what counts.
One important note: sitting a lot is an independent risk factor. Even if you meet the exercise minimum, a sedentary lifestyle for the rest of the day is harmful. The most sedentary people had more than twice the mortality risk of the least sedentary people. Getting enough exercise and sitting less are therefore two separate things to pay attention to.
This answer is based on large studies in which tens of thousands of people were followed for many years, supplemented by multiple systematic reviews of such studies. The association is consistent and robust. Because people were not randomly assigned to exercise more or less, cause and effect cannot be fully proven: people who exercise tend to be healthier in more ways than those who do not. That explains part of the observed benefit, but researchers account for such factors in large analyses, and the association holds up.
Do you live longer from playing tennis or squash?
For tennis there are indications of a substantial gain in life expectancy, but for squash the evidence is entirely absent. If you have to choose based on what is currently known, tennis wins that comparison easily.
Why do some cells stop dividing as you age?
Cells stop dividing because their chromosome tips wear down and they accumulate DNA damage: a protective mechanism that in later life becomes harmful precisely because the stalled cells build up. Treatments that address this are still under active investigation and are not ready for self-application.