I have read most of the replies on this thread. Thought I'd share that I'm going through this same issue right now. First let me say that I sent my DD to Kindergarten when she just turned 5. She is very petitie the size of most 3 year olds and still is. It was hard for her because kindergarten in our district is 7:55 to 2:55 which I think is terribly long school day. She was more than mature enough and was academically were she needed to be. Size is not the issue here at all. She gets A's and B's this year and third garde and school is super easy broing for her. So sending her at 5 was right for her.
Now my DS4 will be 5 this summer. I don't think he is ready to start Kinder in August. He is way too immature and I don't want him to struggle. His learning is right where it should be in my opinion. A few kids in his preschool are ahead with writing & reading skills. The preschool and pediatrician do not think he is mature enough for full day Kindergarten. this is such a struggle making this decision. I want to send because I don't want him to bored when he is 6 and ahead of the other kids. Also him and DD would be in the same school when she is 4th grade and he is Kinder. The following year she will be in middle school for 5th grade, so they would be in two different schools. I honestly don't think he is ready though and I want to do what is best for him. He is the kid who cried at preschool the first 2 weeks and is really quiet most of the time there. I guess well have to see how he does in a few more months.
I would recommend giving him a chance in Kindergarten and if it doesn't work out you can always pull him out. Having peers in Kindergarten can pull that quietness out of him as we have seen it in my dd's class this year with one of her preschool friends. I was so amazed 2 weeks ago when I saw him for the first time in a while and he is just such a different child. He was more outgoing and has made such strides in Kindergarten than he did in preschool.
As for my dd, the teachers still do not believe me when I say she is not shy. They don't see her real side. They see this quiet little girl who barely talks to anyone but her friends and never in the classroom.