Maximal heart rate (HRmax) declines substantially with age, but the magnitude and possible modifying effect of gender, body composition, and physical activity are not fully established. The present study examined the relationship between HRmax and age in 3320 healthy men and women within a wide age range using data from the HUNT Fitness Study (2007–2008). Subjects were included if a maximal effort could be verified during a maximal exercise test. General linear modeling was used to determine the effect of age on HRmax. Subsequently, the effects of gender, body mass index (BMI), physical activity status, and maximal oxygen uptake were examined. Mean predicted HRmax by three former prediction formulas were compared with measured HRmax within 10-year age groups. HRmax was univariately explained by the formula 211 − 0.64·age (SEE, 10.8), and we found no evidence of interaction with gender, physical activity, VO2max level, or BMI groups. There were only minor age-adjusted differences in HRmax between these groups. Previously suggested prediction equations underestimated measured HRmax in subjects older than 30 years. HRmax predicted by age alone may be practically convenient for various groups, although a standard error of 10.8 beats/min must be taken into account. HRmax in healthy, older subjects and women were higher than previously reported.