In countries with a school-entry cutoff date, individuals born right after the cutoff are almost 1 year older than individuals in the same school cohort born right before that date. Abundant research has documented that, as a result of that extra year of maturation and skill accumulation, older students in a cohort outperform their younger peers. It is also well-established that this effect peaks at the initial stages of the educational career and wanes as students grow. However, it remains unclear whether or not the age at school entry affects final educational attainment. In this work, we use Spanish census data to assess whether individuals born right after the school-entry cutoff (January 1) are more likely to complete post-compulsory education, a university degree and post-graduate education. We also assess if the age at school entry affects the probability of completing education in a STEM field of study. Our findings indicate that males born after the cutoff accumulate more years of education than males born before that date, but are less likely to complete their education in a STEM field of study. Interestingly, the effect concentrates among the youngest and oldest students in each cohort, is less intense for higher levels of education and disappears among females.