The answer is generally that children enter kindergarten at or around age 5. This means that if your child is four and will turn five within a certain period of time then he/she will be allowed to enter kindergarten. But, if he/she turns five after the specified date then your child will not begin kindergarten until the following year. Most states have very similar protocol, but may differ somewhat. So, as soon as your child turns four you need to begin looking into when your child is old enough to start kindergarten.
I am answering as a former kindergarten teacher of 30 years. Even when the old deadlines were "5 before Oct. 1", he would not be eligible to attend. Our deadline (Ohio) is now "5 before August 1". If a parent requests early entrance, the child must go through a long test (of which no one has ever passed in my area). From experience as a mother and teacher, please do not push your child into school, especially if your child is a boy. Boys typically mature slower than girls in maturity and fine motor skills. Think ahead also....would you want your child entering college at the age of 17? So much more is now expected of kindergarten children just in the past couple years. I would highly suggest a year at a good preschool for this year - not a daycare, but a preschool with qualified teachers. If this is not an option, there are wonderful websites available. Of course, there are pros and cons to each side of the issue.
Access to Kindergarten: Age Issues in State Statutes
(AR ST § 6-18-201)
5 on or before September 15
(AR ST § 6-18-207) "Any student who has been enrolled in a state-accredited or state-approved kindergarten program in another state for at least 60 days, who will become 5 years old during the year in which he is enrolled in kindergarten and who meets the basic residency requirement for school attendance, may be enrolled in a kindergarten program in a public school in this state" (AR ST § 6-18-207).
The law is different for metropolitan school districts. "All metropolitan school districts. . . may establish and enforce a regulation which requires that a child shall have attained the age of five for purposes of kindergarten and summer school prior to a kindergarten school term. . .on or before any date between August first and October first of that year" (MO ST 160.054).