When a child is younger than most of her classmates be, which is better: To delay her entry into school and give her another year to mature and become "ready" for schooling? Or to enroll her, assuming that she will benefit from the activities and interactions and learn just as much as the older children? Most children in the United States today enter kindergarten at 5 years of age, but many states are considering increasing the age of school entry. Schools face increasing pressure to be accountable for their students' performance, so they are looking for ways to increase children's achievement. If older children benefit more from schooling, they should show higher achievement test scores, which often translate into better funding and resources. Parents are increasingly holding their "younger" children (particularly boys) back from entering school for fear that these children may not learn as well or as quickly as their classmates, perhaps might be retained, and then might have behavioral problems.

Those who support older entry ages argue that older children are biologically,cognitively, and socially more mature, and therefore that they are better able to take advantage of formal instruction. There is also the implicit assumption that the "'gift of time' and general (out of school) experience,outweigh the benefits of a school setting" for increasing readiness (Stipek, 2002, p. 4). Others disagree, arguing that there is no single age at which all children will achieve "readiness" across all the biological, cognitive, and social skills involved in schooling. No matter what the cutoff age is, some children will be more and some less ready. More fundamentally, critics argue that holding children back from schooling will only put them further behind, because the school environment is precisely what children need to further their development. Keeping children out of school will only increase the gap between their skills and those of their peers, and it will put them at even higher risk for poor academic achievement. Finally, critics argue that it is not the child's responsibility to be ready for school, but schools' responsibility to be ready for children—no matter what their level of skills and abilities when they begin.

The research evidence does not argue strongly for older entry ages. Some studies indicate a small advantage for some skills for older children, but the difference fades within the first few years of schooling. For most skills studied, schooling has a significantly stronger effect than age, and younger children at a grade level benefit from schooling as much as older children (Oshima & Domaleski, 2006; Stipek, 2002). There also may be some risks for children who are older than their classmates because of delayed entry. These children show more behavior problems than younger children at the same grade level, with some studies finding that the difference increases over time while others show no long-term disadvantages (Byrd, Weitzman, & Auinger, 1997; Lincove & Painter, 2006; Mayer & Knutson, 1999).