Health-related quality of life (HRQoL) has been characterized as the ultimate goal for health interventions such as physical activity (PA). We assessed how frequency, duration, and intensity of PA were related to HRQoL in younger (<65 yr) and older (≥65 yr) females and males. This population-based cross-sectional study explored associations between frequency, duration, and intensity of PA and physical and mental health. HRQoL was measured by SF-8 Health Survey. Frequency and duration were assessed by items validated in a previous HUNT study, and intensity was assessed by Borg RPE scale. Associations between PA and physical and mental health were estimated using general linear modeling. A total of 4500 participants (56% females), age 19-91 yr, with mean age of 53±15 yr, were included. Of these, 40% were less active than recommended by international guidelines. In general, mean physical health (PCS-8) in females and males was 47.4±9.7 and 48.8±8.9, and mental health (MCS-8) was 50.5±8.0 and 51.9±7.3, respectively. Age-adjusted association between PA and HRQoL was stronger for physical than mental health in both genders and age groups. The largest differences were between no exercise and exercise groups at any level for frequency, duration, and intensity of PA. We found no substantial gender differences in association between PA and HRQoL, but association was stronger in older (≥65 yr) than younger (<65 yr) females and males. Adjusting for socioeconomic factors and factors such as presence of diseases, body mass index, smoking habits, cohabitation, and disablement did not change the results. The study suggests that exercising at any level is associated with better physical and mental health in both genders compared with no exercise, particularly among the older individuals.