National Assessment of Educational Progress data in reading, mathematics, and science for Caucasian children were analyzed. Using regression analysis for three age groups, nine, thirteen, and seventeen year olds, a trend analysis was plotted. The changing achievement relationships between these students relative to their classmates and their age of entry into first grade were studied. The data indicated superior performance for classes with an older combined mean age. The age of entry into first grade proved to be a critical factor in achievement levels. The authors suggest changing entrance cutoff dates from December, January, or February to September, October or November. Since achievement varies between boys and girls, separate cutoff dates were proposed. Clinical screening of students is recommended for critical male and female groups. Teachers' awareness of high risk ages might lead to multi-level instruction. Inadequate readiness is a potentially serious threat to a child's academic career. However, children whose entrance is delayed, although they might have been able to cope, may cause problems as restless high school seniors. Primary type of information provided by the report: Results (Secondary Analysis) (Interpretation). (DWH)