Moderate regular exercise correlates with better health and life expectancy in human epidemiological studies, and is shown in animal studies to be the cause of better health and life expectancy. Here is one of many studies to show that the benefits of exercise continue all the way into old age:
The majority of adults aged 65 and older remains inactive and fails to meet recommended physical activity guidelines, previous research has shown. However, these studies have not represented elders living in retirement communities who may have more access to recreational activities and exercise equipment. Now, [researchers] found that older adults in retirement communities who reported more exercise experienced less physical decline than their peers who reported less exercise, although many adults - even those who exercised - did not complete muscle-strengthening exercises, which are another defense against physical decline.
"Physical decline is natural in this age group, but we found that people who exercised more declined less. The most popular physical activities the residents of the retirement community reported doing were light housework and walking, both of which are easily integrated into individuals' daily lives, but these exercises are not the best choices for maintaining muscle strength."
[Researchers] studied the physical activity of 38 residents at TigerPlace, an independent-living community in Columbia, four times in one year. The researchers tested the residents' walking speed, balance and their ability to stand up after sitting in a chair. Then, researchers compared the results of the tests to the residents' self-reported participation in exercise. [Residents] who reported doing more exercise had more success maintaining their physical abilities over time.