The focus of this research was to determine the relative age effect (RAE) on selection in the Slovakia national football teams. A factor that may have a significant impact on the quality of players chosen for the national teams or may result in a poor selection of players for the elite teams. Anthropometric and cognitive acceleration of players born in the first months of the calendar year concerning the overall context of the competition for placement in the national teams may be considered as a significant advantage. The aim of this research was to examine, determine and verify the presence of relative age effect in the selection of football players for the Slovakia national teams starting with the under 16 age category (U-16) through to the A - senior national football team. We presumed that the elite teams under this review and study consisted predominantly of players born in the first quarter of the calendar year, while also presuming that relative age effect receded with the increasing age category. Our survey sample U16 consisted of 79 players, U17 consisted of 47 players, U18 consisted of 58 players, U19 consisted of 71 players, U21 consisted of 52 players and A - senior national team consisted of 302 Slovakia national football players. The information obtained from the Slovak Football Association has been processed by the application of statistical methods and statistical significance test (T-test). Our research confirmed the presence of relative age effect in the U-16, U-17 and U-18 teams under our investigation (p≤0.01). In the U19 and U21 age categories, statistical significance has not been confirmed. As for the senior national team, statistically significant difference has been found in relation to players born in the last quarter of the year as opposed to players born in the first three months of the year (p≤0.01). Our results have shown that with the increasing age, the relative age effect fades and vanishes in full in the category of senior elite players. In the U-19 and U-21 age categories, statistical significance has not been confirmed. Based on this research results it may be recommended to use the advantage of relative age effect for the selection of players of the particular age category in the particular competition (such as e.g. the European Championship qualification, the European Championship final tournament qualification, or the World Cup qualification in the U-16, U-17, U-18 age categories).