Public health surveillance systems often monitor physical activity trends, but fitness assessment is relatively rare. This study investigated secular changes in aerobic fitness among Canadian adults and children. Participants aged 8-69 years were from 2 nationally representative surveys, conducted in-home in 1981 and in mobile examination centers in 2007-2009. In both surveys, submaximal step tests using progressive age- and sex-specific exercise stages were completed after initial screening (Physical Activity Readiness Questionnaire, heart rate, blood pressure). Between surveys, the step-test protocol had been modified to reduce underestimation of fitness among fitter and older individuals. Maximal oxygen uptake was estimated for adults using validated historical and updated prediction equations, adjusted to reflect protocol differences. Because these equations are not validated for young people, maximal aerobic power was predicted at a heart rate of 200 beats·min(-1) by regressing observed heart rates on the oxygen costs of stepping for children and youth who completed at least 2 exercise stages. Overall, despite protocol differences, we found that the aerobic fitness levels of Canadians were lower in 2007-2009 than in 1981, with declines apparent in all age and both sex groups, thereby increasing the number of those at risk of adverse health outcomes. Future work is required to validate prediction equations of aerobic fitness for young people to make it possible to compare fitness levels over the lifespan and across time.