This might seem like an easy question but for parents it is one that they have to consider depending on where they live. More and more middle class and upper class parents are keeping their children out of schools until their child is older. The rationale is that 1. their child will be one year or more older than their peers so they will be one year more mature and compete at a higher level for scholarships and academic honors. 2. their child will be more physically mature and compete at a higher level in athletics. 3. that they live in an area where everyone else is keeping their children out of school an extra year and so if they send their child at 5 years old their child will be significantly younger than the rest of their class.
On the other end in some communities are those parents that see the school as taking care of their child care concern because their are not parents at home and they have been paying for child care. School districts are even getting into the act by making the list of skills and knowledge tat children must have to enter kindergarten similar to what children knew and could do when leaving kindergarten.
It is important for parents to learn something about their school district. Ask about the age range and average age of children entering kindergarten for the prior three years. Talk with your child's preschool teacher (these days children who do not attend some kind of out of the home preschool experience are often found to be behind other children socially and in their independence and problem solving abilities). Look at a typical kindergarten readiness test and judge for yourself how you think they measure up.
The time to start thinking about these issues is in January and not in April or May as the testing starts. With most of the areas on the kindergarten readiness tests, time is not the answer. Children do not just get more knowledgeable and skilled just because they are more mature. There may be work for you all to do.