Greater physical activity correlates well with lower mortality in later life. Given the way that human data is collected, and the way in which epidemiological studies are carried out, it is hard to determine causation, however.
Is it that exercise is protective, or is it that more robust people both live longer and exercise more often?
Fortunately the equivalent animal studies on exercise are unambiguous, and show that exercise does in fact act to improve long-term health and reduce premature mortality. Here, researchers expand on the existing evidence by focusing on trends in physical exercise in later life, and how those trends correlate with mortality. They find the expected outcome, in that a reduction in exercise over time is worse than the alternatives.
Source: Fight Aging!