Does the mathematical reasoning ability develop with increase in age? How is mathematical reasoning ability differing according to gender? The current study is trying to find answers to these two questions. The study using cross-sectional design, was conducted with 409 (8th, 9th and 10th grade) students attending middle school and high school in different provinces of Turkey from different socio-economic environments. Mathematical Reasoning Test (MRT) was used for the data collection. Independent groups t-test was applied in order to analyse the relationship between mathematical reasoning and gender, and additionally ANOVA test was used to determine the differences between grade levels. The analysis shows that as the age increases mathematical reasoning develops and male students perform significantly better than female students in mathematical reasoning. It is very important to (a) take encouraging steps to ensure that women are interested in mathematics instead of discouraging attitudes in society, and (b) expose students to higher-level problems in open-ended format without answer options over grades in order to improve their mathematical reasoning.