The Relative Age Effect has been known for quite a while now. This is a bias, evident in the upper echelons of youth sport but also in education and academia, where participation is higher amongst those born early in the relevant selection period (and correspondingly lower amongst those born late in the selection period) than would be expected. This chart that I found via Wikipedia makes it clear when discussing sport.
Also in education this is a controversial issue as Zhong notes “…because some parents now deliberately enrolled their children a year after they are eligible to start school, in the hope of giving them a permanent learning advantage. This is known as redshirting.” This redshirting might be a better option than retention. New British research looked not at the benefits of being older in the classroom, but the negative effects of being younger. The youngest children in each school cohort are overrepresented in referrals to mental health services compared with older children in the same cohort.
Abstract of the research:
To investigate whether the youngest children in each school cohort are overrepresented as users of specialist mental health services.
Dates of birth were obtained for all 9,157 children and adolescents referred to specialist mental health services in 3 London boroughs from 2008 to 2011. The actual frequency of referrals by month of birth is compared to the expected frequency of referrals as determined by birth statistics for the relevant age group.
August-born children, who are the youngest in their cohorts in England, represent 9.38% of referrals but only 8.59% of the population in the relevant age segment. Hence, August-born children are overrepresented in referrals to specialist mental health services (P value = .007). September- and October-born children, who are the oldest in their cohorts, are underrepresented: September-born children represent 8.62% of the population but 7.99% of referrals to mental health services (P value = .032), and October-born children are 8.56% of the population but 7.86% of referrals (P value = .016).
Being among the youngest in a school cohort is associated with a higher risk of referral to mental health services, while being among the oldest is a protective factor.