Political selection is crucial for the functioning of democracy. However, the practice—in education and sports contexts—of artificially dividing school‐age children into different age groups leads to a considerable bias in this selection. The probability of becoming a (successful) politician depends on individuals' relative age. Being born shortly after the cut‐off date significantly increases the probability that an individual will be politically successful later in life. Using a regression discontinuity design, we find strong evidence of such relative age effect (RAE) among a large sample of Belgian federal parliamentarians over the period 1950–2019 (N = 4032), but not among municipal councillors (N = 7387), nor among municipal candidates (N = 36,740) in the 2018 election. The estimated overrepresentation of federal members of the parliament (MPs) born immediately after the cut‐off date is up to 90% compared to politicians born just before the cut‐off date. The overrepresentation is observed over the whole period and thus seems to be deeply rooted in the political system. We find the RAE to have a gendered dimension: The effect is driven by early‐born male politicians' overrepresentation. No significant RAE was found among female politicians.