Objectives: To determine if the association between frequency of leisure-time physical activity and mortality risk differs across adulthood. Method: 9,249 adults from the NHANES III (1988-1994) were categorized as middle-aged (40-64 years), old (65-79 years) or very old (≥80 years), and as inactive (0 bouts of physical activity/week), lightly active (1-2 bouts/week), moderately active (3-4 bouts/week) or very active (5+ bouts/week). Results: In all age categories, lightly, moderately, and very active adults had a lower mortality risk compared to inactive adults (p < .001). In very old adults only, being very active was associated with a lower mortality risk compared to being lightly active (HR 0.80, 95% CI 0.64-0.98; p = .03) and moderately active (HR 0.80, 95% CI 0.65-0.98; (p = .03). Discussion: The association between physical activity frequency and mortality risk is strongest in very old adults. All adults and particularly very old adults may benefit from participating in physical activity five or more times a week.