The U.S. Census collects data on school enrollment by age, and the file
http://www.census.gov/hhes/school/data/cps/2009/tab02-01.xlsTable 2. Single Grade of Enrollment and High School Graduation Status for People 3 Years Old and Over, by Sex, Age (Single Years for 3 to 24 Years), Race, and Hispanic Origin: October 2009
tells you that 78,000 3-year-olds were enrolled in KG in 2009 (compared to 3,068,000 5-year-olds). Similar data is given up through the first two years of graduate school.
Not sure what to do with this data, just think it is interesting.
