This paper reports on research conducted to determine whether there is a statistically significant difference between the achievement test scores of students who enter first grade as 5-year-olds, and those who enter first grade as 6-year-olds. Ex post facto research was conducted on 3 samples of students (17,000 from 1st grades, 17,500 from 4th grades, 19,450 from 8th grades) in Kentucky schools. Pupil test scores for each sample on the Comprehensive Tests of Basic Skills (CTBS) total reading, total math, total language, and total battery were compared. A t test for independent samples was applied to each of the four kinds of scores (reading, math, language, battery) for the three different grade levels. In addition, each sample was further subdivided into two groups according to sex in order to determine if probable achievement differences at all three levels were consistent among both boys and girls. Results indicate that students entering the first grade as 6-year-olds had significantly higher achievement scores at both the 1st grade and 4th grade levels than did students who entered 1st grade as 5-year-olds. At the 8th grade level, 6-year-old entrants scored higher in only one area (reading), and findings seem to indicate that the difference observed in 8th grade reading scores may be more a function of sex than of first grade entrance age. Numerous tables are included. (CM)