what age is required?

I know in washington, it has to do with the timeline of kindergarten as a class. It was originally offered as a service, and not all school districts offered it and not all parents took them up on it. There hasn't been a need to change that because it hasn't been a problem. Most parents want the free day care with certified instructors (ie teachers), and those that don't still have kids that are decently prepared for first grade (well, at least as well as the kids who had kindergarten. Some 5 and 6 years just aren't ready to pick up pre-academic skills).
It definitely varies by state, just like minimum ages for kindergarten. In the south, you have to be 5 by July 31, but in WA state, you have to be 5 by September 1st (or October 1st). When kids move states, they can be a year older or younger than is possible in their "home" state. It makes a big difference in what the teachers expect from the kids, esp. in younger grades. For instance, just turned 5's have more potty accidents than just turned 6's. It's just human growth, ya know? Ohh. I've gone way off topic here.
Yes, look it up for your state.
Oh, and CPS does take a dim view of "forgetting" how old your kid is. You can chose to not do kindergarten and not do anything academic or do a private school, but you can't forget they are now 5. That looks sloppy.
And, yes, every year there are kids that get enrolled in March because the neighbor said something to the parents and the parents had totally forgotten.

Similar topic over at diaperpin and I did some research. Quick copy and paste of my answer for you:

Compulsory school ages according to the hslda. See http://www.hslda.org for more complete details on your state:

Alabama 7-16

Alaska 7-16

Arizona 6-16

Arkansas 5 through 17 on or before September 15 of that year"; a child under age 6 on September 15 may be waived from kindergarten with submission of a state-provided form

California 6 by December 2

Colorado 7-16

Connecticut five years of age and over and under sixteen years of age" five- or six-year-olds can opt out when the parent goes to the school district and signs an option form

Delaware "between 5 years of age and 16 years of age"; can delay start (if "in best interests of the child") with school authorization

District of Columbia age of 5 years by December 31 of current school year

Florida attained the age of 6 years by February 1

Georgia between 6th and 16th birthdays"; a child under 7 who has attended public school for more than 20 days is also subject to the compulsory attendance law

Hawaii have arrived at the age of at least 6 years and...not...at the age of 18 years by January 1 of any school year"

Idaho attained the age of 7 years, but not the age of 16 years

Illinois between the ages of 7 and 16 years

Indiana Earlier of the date on which the child officially enrolls in a school or reaches the age of 7 until the date on which he reaches the age of 18

Iowa age 6 by September 15 until age 16

Kansas reached the age of 7 and under the age of 18 years

Kentucky has reached 6th birthday and has not passed 16th birthday

Louisiana from the child's "7th birthday until his 18th birthday

Maine 7 years of age or older and under 17 years

Maryland 5 years old or older and under 16" with one-year exemption available for 5 year-olds

Massachusetts 6 (by December 31 of that school year) to 16 years of age

Michigan age of 6 to the child's 16th birthday

Minnesota between 7 and 16 years of age

Mississippi age of 6 on or before September 1... and has not attained the age of 17 on or before September 1

Missouri between the ages of 7 and 16 years

Montana 7 years of age or older prior to the first day of school" and "the later of the following dates: the child's 16th birthday; the day of completion of the work of the 8th grade

North Carolina between the ages of seven and 16 years

North Dakota a child between the ages of seven and sixteen years

Nebraska not less than 7 nor more than 16 years of age

Nevada between the ages of 7 and 17 years

New Hampshire at least 6 years of age [on September 30] and under 16 years of age

New Jersey between the ages of six and 16 years

New Mexico at least five years of age prior to 12:01am on September 1 of the school year" to the "age of majority" unless the person has graduated from high school; children under eight can be excused

New York a minor who becomes six years of age on or before the first of December in any school year...until the last day of session in the school year in which the minor becomes sixteen years of age" or completes high school

Ohio between six and eighteen years of age

Oklahoma over age of five (5) years and under the age of eighteen (18) years

Oregon between the ages of 7 and 18 years who have not completed the twelfth grade

Pennsylvania From time the child enters school, "which shall not be later than the age of eight (8) years, until the age of seventeen (17) years

Rhode Island completed six (6) years of life on or before December 31 of any school year and not completed sixteen (16) years of life

South Carolina five years of age before September first until . . . seventeenth birthday or" graduation from high school; five-year-olds may be excused from kindergarten with submission of written notice to the school district

South Dakota six years old by the first day of September and who has not exceeded the age of sixteen years"; children under age 7 can be excused

Tennessee between the ages of six (6) and seventeen (17) years, both inclusive"; also applicable to children under age 6 who have enrolled in any public, private, or parochial school for more than six weeks; a parent of a six-year-old may make application for a one-semester or one-year deferral with the principal of the public school in which the child would be required to attend

Texas a child who is at least six years of age, or who is younger than six years of age and has previously been enrolled in first grade, and who has not yet reached the child's 18th birthday

Utah a child who has reached the age of six years but has not reached the age of eighteen years

Vermont between the ages of six and 16 years"; children attending a post-secondary school (approved or accredited by Vermont or another state) are exempt

Virginia have reached the fifth birthday on or before... September 30...and who has not passed the eighteenth birthday; 5 year-olds can be excused

Washington eight years of age and under eighteen years of age

West Virginia compulsory school attendance shall begin with the school year in which the sixth birthday is reached prior to the first day of September of such year or upon enrolling in a publicly supported kindergarten program and continue to the sixteenth birthday

Wisconsin between the ages of 6 [by September 1] and 18 years

Wyoming whose seventh birthday falls before September 15 of any year and who has not yet attained his sixteenth birthday or completed the tenth grade

HTH's

Kris