In the state of Oregon, a child needs to be 5 on or before September 1 to start kindergarten. However, attending kindergarten is not required. Below is a link of all the states kindergarten entrance date and whether it's mandatory.
Although, Kindergarten may not be required, it is so beneficial to the child preparing them for the much needed skills for grade school. Also, a student can fail Kindergarten (even in a state that it is not required) the child will be refrained in order to better prepare the student.
I can understand not wanting to enroll a child in preschool due to young age and they only teach the basics that you can easily achieve at home, such as colors, shapes, alphabet, name, etc., but this prepares the student to be successful in Kindergarten and if a parent is in a situation which disables them to prepare their child at home, it is a great alternative. However, Kindergarten is a different concept all together and it will determine if a child gets off on the right foot or not. Attendance of a Kindergarten student is exactly the same as grade school and it really helps them to adjust to the school, the schedule, the policies and rules, following instructions, getting to know their classmates and just overall prepares them to perform adequately in 1st grade.
Required or not, I would consider the benefits of enrolling my child in Kindergarten. These may be links that you will be interested in.