The term relative age effect (RAE), also known as birthdate effect or birth date effect, is used to describe a bias, evident in the upper echelons of youth sport  and academia, where participation is higher amongst those born earlier in the relevant selection period (and lower for those born later in the selection period) than would be expected from the distribution of births. The selection period is usually the calendar year, the academic year or the sporting season.
The difference in maturity often contributes to the effect  with age category, skill level and sport context also impacting the risk of the relative age effect. Mid to late adolescent, regional to nation, popular sports seeing the highest risk, and under 11, recreational, unpopular sports seeing the lowest risk.
The terms month of birth bias and season of birth bias are used to describe similar effect but are fundamentally different. Season of birth examines the influence of different prenatal and perinatal seasonal environmental factors like sunlight, temperature, or viral exposure during gestation, that relate to health outcomes. Whereas the relative age effect shifts with selection dates  moving the advantage with the selection period. With influence from social agents  children born soon after the cut-off date are typically included, and a child born soon before the cut-off date excluded.