The relative age effect is regarded as a contributing factor to sporting success. For example, the Federation Internationale de Football Association (FIFA) uses a system for youth soccer with January 1 as the cut-off date to establish its age groups. Within the same age category, a difference of almost one full year may exist between the oldest and youngest participants. Therefore, relatively older children within a particular age group are more likely to achieve sporting success. This phenomenon has been called the relative age effect. Relatively older children have advantages in growth, biological maturity, and cognitive development . In addition, relatively older children (athletes) have a greater opportunity to participate in competitions and, consequently, may enhance their psychological, technical, and tactical abilities, thereby supporting greater athletic development . The relative age effect has been confirmed in many types of sports, including baseball [3, 4], soccer [5,6,7,8], tennis , cricket , basketball [4, 11], NASCAR , sumo wrestling , rugby , judo , ice hockey [14,15,16,17], and winter sports [18,19,20,21].
Moreover, several studies have examined the relative age effect from a historical perspective [8, 16, 22,23,24,25]. It generally takes several years or decades for a sport to gain popularity in a given country. Thus, historical analyses are needed in order to clarify the beginning of the relative age effect in a country and compare differences in the skew of this effect among generations.
The present study focused on how long the relative age effect continues into adulthood because most studies have focused on junior players, while, to the best of our knowledge, only a few studies have examined this topic. We previously reported that the relative age effect persisted among players older than 22 years of age when, theoretically, no physical advantage is expected for older players . The relative age effect has been demonstrated in professional athletes who graduated university (college) at 22 years old; however, this relationship was weaker than that among those who graduated high school at 18 years old. Steingröver and colleagues  recently investigated whether relative age influenced career lengths in the National Basketball Association (NBA), National Hockey League (NHL), and National Football League (NFL). They showed that the number of matches played was significantly larger in relatively younger players than in relatively older players in the NHL. No significant differences were observed in career lengths in the NBA or NFL between relatively younger and older players.
The present study examined the relationship between the relative age effect and lengths of professional careers among professional male Japanese baseball players. Steingröver and colleagues  reported significant differences in career lengths between relatively younger and older players in the NHL; however, this relative age effect needs to be confirmed in other countries if universal factors are truly related to this effect. In other words, even if a significant relative age effect is observed in a country, the popularity and system of a sport differ among sports and countries. In Japan, a unique annual-age grouping has been applied since 1886, which is between April 1 and March 31 of the following year. Therefore, April 1 is the beginning of the “new year” (i.e., cut-off date), and this specific calendar follows an education system including elementary, junior high, and senior high schools and university (college), government, and companies. Sports calendars also follow this system. Thus, players born in April, May, and June are expected to have a relative age advantage. Grondin and Koren  reported that the relative age effect for baseball was more important in Japan than in the USA because large numbers of Japanese players were born during Q1 (April–June). Based on these backgrounds, a relative age effect was hypothesized to exist on the lengths of professional careers among Japanese professional male players.