Purpose: To estimate the association of age with maximal HR (MHR). Methods: Data were obtained from the Coronary Artery Risk Development in Young Adults (CARDIA) study. Participants were black and white men and women aged 18-30 yr in 1985-1986 (year 0). A symptom-limited maximal graded exercise test was completed at years 0, 7, and 20 by 4969, 2583, and 2870 participants, respectively. After exclusion, 9622 eligible tests remained. Results: In all 9622 tests, estimated MHR (eMHR, bpm) had a quadratic relation to age in the age range of 18-50 yr, eMHR = 179 + 0.29 × age2-0.011 × age2. The age-MHR association was approximately linear in the restricted age ranges of consecutive tests. In 2215 people who completed tests of both years 0 and 7 (age range = 18-37 yr), eMHR = 189-0.35 × age; and in 1574 people who completed tests of both years 7 and 20 (age range = 25-50 yr), eMHR = 199-0.63 × age. In the lowest baseline body mass index (BMI) quartile, the rate of decline was 0.24 bpm•yr between years 0 and 7 and 0.51 bpm•yr between years 7 and 20, whereas in the highest baseline BMI quartile, there was a linear rate of decline of approximately 0.7 bpm•yr for the full age range of 18-50 yr. Conclusions: Clinicians making exercise prescriptions should be aware that the loss of symptom-limited MHR is much slower in young adulthood and more pronounced in later adulthood. In particular, MHR loss is very slow in those with the lowest BMI younger than 40 yr. © 2010 by the American College of Sports Medicine.