DS had to be 5 by September 1st to start K, he turned 5 on August 18th. We debated an on time start or a late start (age 6). Ultimately we started him t 5 and while it has had his bumps, we are all glad he started then. He is in 2nd grade and has tons of friends, is in the top reading and math group. He is working on 3rd grade and higher material. His behavior is sometimes on the young side but part of that is his personality. He is extremely out spoken, independent thinking, and very high energy...none of which tend to be valued traits by school teachers.
Factors that helped us make the decision...
1) DS wanted to start at age 5
2) I was available to volunteer in the classroom a lot which eased the transition for him. Not all schools allow this though.
3) Very bright and would have been academically bored if we held him back
4) Lots of parental support at home to help him process social-emotional challengees as well as arrange for special therapies to help with weak fine motor skills before it became too big of a discouragement for him.
5) DS was worried about being gone all day (he had done 3 mornings a week for preschool) so he wanted a chance to practice. We signed him up for 2 weeks of an all day academic summer program. It was invaluable to him and us to see how he handled the schedule and to figure out what worked to help him succeed each day..
6) We registered for both preschool and K. That way we didn't feel pressure to make the decision when he was only 4.5yo. He changed a lot in the next 6 months and we were able to make a better decision in August. We actually waited until the day before school started to make the final decision.