'Age is just a number'. 'You're only as old as you feel'. While these statements are thrown around consistently, we can't ignore science and human physiology. Our immune system starts to decline 1-2% from our 20s making older individuals more susceptible to infections, cancers and certain diseases/ conditions.
*Link below* is a study that shows how exercise helps prevent the immune system from declining giving you added protection to infections. This study looked at 125 cyclists and, using T cells as immune markers, found that participants in their 70s and 80s had the immune systems of participants in their 20s! A separate study, which used the same participants, found these older individuals did not lose muscle mass or strength and did not see an increase in body fat (things that are typically associated with old age). Perhaps my favorite quote from the researcher "if exercise was a pill, everyone would take it"!
Walsh, Ferguson. 'How exercise in old age prevents the immune system from declining. BBC News, 8 March 2018. Retrieved from http://www.bbc.com/news/health-43308729