The aim of this study was to examine the performance of different age groups of students in mathematical achievement, i.e. in solving mathematical problem tasks and mathematical school grades. Furthermore, the study aimed to investigate the contribution of the mathematics self-concept and subjective value of Mathematics to explain individual differences in solving mathematical problem tasks and school achievement in Mathematics. A total of 780 participants took part in the study. The results show that with age school achievement in Mathematics decreases, as well as Mathematics self-concept and students? subjective value of Mathematics. On the other hand, older students are more successful in solving mathematical problem tasks. However, the performance in mathematical problem solving was relatively poor throughout the sample. Although female students had better grades, there were no significant gender differences in relation to solving problem tasks or mathematical self-concept and the subjective value of Mathematics. Younger students, in comparison to older ones, assessed their mathematical abilities higher, were more interested in Mathematics and evaluated it as more useful. The only commonly important predictors of both problem solving tasks and most frequent grade in Mathematics were age and the self-assessment of mathematical abilities.