S.,
As a mother, who started her daughter as a young five (Sept. birthday) to a school Principal who saw both young and older kindergarteners, to a grandmother who waited until my grandson was 6, I feel I have a perspective to share with you.
In today's world, while society and "Educators" want to tell you that you should have your child in structured learning by 3 or 4, I will tell you the truth. Your child needs you to be their tether until they are old enough to handle the weight of society. That is not to say you cannot expose them to classes or groups, but they need you.
My position is that our children today are faced with pressures and decisions that we didn't have until much older, even at the kindergarten level. They need to be equipped in character (which is taught at the home) to be able to successfully navigate the cesspool out there.
Little boys develop somewhat behind girls physiologically, so that alone places them behind the curve if you start them too young. Their dexterity and focus development alone will be a frustration factor with rare exception.
While I taught my grandson phonetics and reading as well as critical thinking, (He and my daughter lived with me) I didn't start actual kindergarten with him until he was six. He turned six in June and he began K the following Sept.
It has been a good decision, for now he has had great success and feels good about himself. We opted to home school, but the principle is the same regardless.
I hope you will listen to that wisdom inside of you. The fact that you are questioning it is telling.
I hope it is clear to you soon, and I am happy to share the truth...