Mathematical anxiety is a psychological construct characterized by feelings of tension, apprehension, or fear that interfere with mathematical performance. It affects learners across all educational stages and has been shown to influence both motivation and academic achievement. This study examines whether sex and age account for differences in levels of mathematical anxiety among Spanish primary school students. A quantitative, correlational design was employed with a convenience sample of 185 students in Grades 3 to 6 of primary education (aged 9-12 years). Mathematical anxiety was assessed using the validated Abbreviated Math Anxiety Scale (AMAS), which is a standard instrument frequently used in the specialized literature. The findings indicate that, on average, students exhibit a moderate level of mathematical anxiety, with no significant differences between sexes. However, age showed a positive and statistically significant correlation with mathematical anxiety. Specifically, anxiety levels decreased until approximately age 10, after which they increased progressively with age. The relationship between age and mathematical anxiety was best captured by a quadratic model. These results suggest the need for targeted interventions to prevent and mitigate mathematical anxiety, with particular emphasis from the age of 10 onwards. Moreover, sex-related differences in the perception of mathematics may become more pronounced in later stages, coinciding with the onset of adolescence.