There is continuing controversy about the optimal or appropriate age at which children should start school. The purpose of this study is to examine the relationship between age and achievement. It is an attempt to evaluate the hypothesis that older students fare better academically than their younger classmates. Findings indicate that on average for students in elementary school there is positive linear relationship between age and achievement for age normal peers. Even though there is positive linear relationship, the difference in average test scores between the oldest and youngest students is not great and by the time students reach 10th grade the positive linear relationship has disappeared. For overage students there is on average a negative linear relationship between age and achievement at all grade levels. That is, the negative relationship between age and achievement remains constant over time. These results argue against modifying entrance age policies, delaying school entry, implementing transitional kindergarten or first grade programs or retaining students to improve educational achievement. Policies and practices that make students older than their classmates inversely affect their educational achievement.
In other words, they found that kids who started kindergarten at the age-appropriate 5 years old, do better if they are on the older side of their cohort by a small amount--the oldest 5 year olds do best. But for kids who start at the age of 6, the older they are the worse they do--the oldest 6 year olds do the worst.
Here are two graphs which show this. The first is a reading test administered in grade 2. The blue highlighted areas (strange, they were yellow on my original, but Blogger changed the color) represent my nephew's June birthday. The one on the left is if he enters kindergarten at age 5, and the right at 6.
You can see that on average, his cohort of like-aged, old-for-grade kids score fully 10 points lower than the kids who start kindergarten on time--52 vs 42, a drop of nearly 20%.
Math is similar:
This one shows that boys like my nephew can expect to have 9 points lower scores than the kids who started a year earlier.
Keep in mind, that these are not tests administered in kindergarten and do not show the childrens' readiness for school. These are tests administered several years into school in second grade. According to redshirt theory, kids like my nephew should be doing better than if they started the previous year, not worse.
So much for second grade, what about later? Above, the studies showed that in second grade, kids who are in the normal grade for their age, but who are on the older side of their cohort do better, than the younger ones. The study showed that as time goes by, this age effect declines considerably, with all the kids basically evening out by the time they leave high school.
The opposite effect occurs for over-age kids. As they go through school, they fall further behind their peers.
For overage students there is on average a negative linear relationship between age and achievement for all grade levels. On average, overage students do less well than their classmates and the older they get the less well they perform. The negative relationship between age and achievement remains constant over time.
[...] Contrary to parental beliefs and educational policy, the results from this study argue against modifying entrance age policies, delaying school entry, implementing transitional kindergarten or first grade programs, or retaining students to improve educational achievement. Proponents of these practices argue that they will improve educational achievement. These results do not support that argument. In addition, although it may be difficult to argue that being overage causes lower achievement, policies and practices that make students older than their age normal peers seem to inversely affect their educational achievement. When students are one year older than their classmates, their average academic performance declines and continues to decline the older they get. Maybe making students different (i.e., older than their grade level peers) lowers their motivation to achieve. The research literature suggests that older students also are more likely to drop out of school.