This study showed that among relatively younger adolescents, females were less likely to participate in sports activities, although both males and females spent less time on MVPA. These results mostly support previous studies indicating that relatively younger adolescents have less time to play sports . Other previous studies have showed that relatively younger adolescents are likely to have poorer physical fitness [6, 7, 33] and noncognitive skills (e.g., self-efficacy, ability belief) [2, 3]. Considering these previous studies, relatively younger adolescents might be more likely to be reluctant to play sports. Although this study did not investigate adolescent sports skills and subjective perception about sports, the capability and interest caused by relative age might have an influence on adolescent sports activity. In addition, we found a relative age effect of organized sports participation only among females. Although the reason for this is not clear, there are some possible explanations. First, since females are likely to have secondary sexual characteristics [34, 35], fewer females might participate in sports activities than males. For example, a female’s physical performance (e.g., running, jumping) reaches its peak at the age of 13, while a male’s physical performance shows a linear improvement . Second, while many males are likely to naturally participate in organized sports activities, enjoying some sports activities may be an important factor for females in engaging in sufficient physical activity . Since relatively younger females might have fewer opportunities to play active roles in sports, they might be less likely to participate in sports activities.
We also observed a significant interaction between MVPA and average annual income for both males and females. Relatively younger adolescents were significantly associated with spending less MVPA time only in low-income neighborhoods. This result indicates that the relative age effect of physical activity might be more likely to emerge in socioeconomically disadvantaged neighborhoods. Although no study has examined how socioeconomic factors moderate the association between birth month and adolescent physical activity, there are some possible explanations. High-income families may afford to spend on recreational activities. Socioeconomically disadvantaged neighborhoods are less likely to have parks [12, 13] or sports facilities [36–38], thus offering limited opportunities to participate in sports activities. Relatively younger adolescents in socioeconomically disadvantaged neighborhoods might have few opportunities to transfer to other sports activities, while relatively younger adolescents who live in affluent neighborhoods might be able to transfer to other sports activities more easily.
Thus, as mentioned above, physical activity is an important factor that mitigates relative age effect . However, since relatively younger adolescents might have undergone fewer successful experiences of sports than their relatively older counterparts, they might likely become inactive. Therefore, teachers and coaches should encourage relatively younger adolescents to engage in physical activity. For example, teachers and coaches must formulate sports rules to enable equal active roles among adolescents. In addition, it is essential for schools and sports facilities to provide later-born adolescents with adequate opportunities to engage in physical activity and to play active roles in sports by participating in various sports. In this study, we observed a relative age effect of physical activity in both male and female adolescents from socioeconomically disadvantaged areas. Socioeconomically disadvantaged adolescents are less likely to engage in physical activity owing to their financial instability. Thus, schools and public sports facilities should support socioeconomically disadvantaged adolescents to engage in sports at minimum financial costs.
This study was the first to examine how socioeconomic factors moderate the association between birth month and adolescent physical activity. In addition, we defined school districts as units of neighborhood, and we applied municipality-level and block-level data for neighborhood-level data. Furthermore, we showed the relative age effect of adolescent organized sports participation and physical activity by adjusting various individual and neighborhood characteristics. Nevertheless, this study has some limitations. First, this was a cross-sectional study; therefore, we could not address causal relations. Second, while some previous studies examined physical fitness as a relative age effect outcome [6, 7, 33], we did not research participants’ physical fitness. However, previous studies have reported that high-level physical fitness is related to more MVPA time . Thus, those who spent more time on MVPA seemed to have a higher level of physical fitness in this study. Third, since we could not ask participants about their SES, we could not address how high the SES of the household is. In Japan, cultural norms make it difficult to directly ask for parents’ SES. Therefore, we substituted neighborhood-level socioeconomic factors based on demographics, such as census data. Previous studies have reported that individual-level socioeconomic factors are more strongly related to adolescent physical activity than neighborhood-level factors [9, 10]. We might observe a clearer association between socioeconomic factors and adolescent physical activity upon utilizing individual-level socioeconomic factors.