The present study evaluated the influence of age and sex on the level of health-related fitness components in secondary school students. The participants were 330 secondary schoolchildren (158 boys and 172 girls) 12-16 years old. For the evaluation of health-related fitness (aerobic capacity, muscular strength and endurance, flexibility and body composition), the Fitnessgram test battery was used. Results revealed significant differences for age and sex for the one mile run-walk test, for the curl up test and for VO2max. Also, there was significant main effect of age on body mass index and the trunk lift test. Moreover, there was a significant main effect of sex on back saver sit and reach test, while no significant differences were found between age and sex in the push-up test. The students succeed in achieving the criteria, which were in the “Healthy Fitness Zone”, as these indicated by the Fitnessgram test battery, in a percentage more than 97% for the curl up test, 88% for the trunk lift test, 78% for the back saver sit and reach test, 72% for the 90o push-up test . In contrary, the students were in the zone “Need Improvement” in a percentage of 39% for the one mile run-walk test, 20% for the VO2max and 25% for the Body Mass Index. A better understanding of the individual differences that exist in different ages and between boys and girls, could have strong implication for planning and teaching physical education in schools aiming to increase students’ fitness level.