The effects of age at entrance into school on subsequent elementary school performance were studied in a cohort of children who started grade 1 in the Dallas school system (Texas) in fall 1981. Most of the subjects were born on or after September 1, 1974 and before September 1, 1975; the spread of ages was 1 year. A total of 3,028 male and 3,019 female students was grouped into four seasonal categories based on time of birth. The subjects were between 6 and 7 years old when they entered grade 1. The Iowa Tests of Basic Skills were administered routinely to all of the students in all grade levels over the 6 years studied (1982 through 1987). In both mathematics and reading, older children began with an advantage over younger children and maintained this advantage through the 6 years studied. The advantage did not appear to be that of an accelerated learning rate, but rather that of a constant difference through elementary school. The performances of the oldest boys were comparable with those of the youngest girls, suggesting that maturation and development played a major role in the differential effects due to age. However, when the seasonal groups were compared, children entering grade 1 between the ages of 6 years 6 months and 6 years 9 months out performed those entering grade 1 between the ages of 6 years and 6 years 3 months; this difference remained constant and cannot be explained on the basis of maturation. Five tables and 10 graphs present study data. (SLD)