My moms position 80% of children not 5yrs old in September for start of kindergarten had lower scoring and learning time adjustment. Her opinion had absolutely nothing to do with sports. Probably for the reasons you state; pre school kids with no early introduction to reading and learning skills, but that is the majority kids coming to school to learn the alphabet and numbers. If your too young(immature) your not sitting and learning your just fidgeting and adjusting to new surroundings and nothing is absorbed. Correct if learning was actually a family practice before kindergarten we would be way ahead, its more and more parents waiting for kindergarten so they can get rid of their kids for the day and have someone else teach them.