My son turned 5 in July, and he is attending what our school district calls "Transitional Kindergarten." It's basically an extra year so that he'll begin kindergarten when he's 6. I tell him he's in the 2 year kindergarten.
I sent him to preschool when he was 3. They had a 3's class and a 4's class. He was by far the smallest kid in the class, but he was happy and didn't seem to notice or didn't care. He loved it, he loved his teachers. It was a great fit. As he began his 2nd year, I kept noticing little things with his peers that made me wonder if we were going to have a maturity issue. He's always been very bright, but socially he just looked like the youngest.
I really thought it would be best to find another school in town that offered a PreK (other than the public schools, because I found out that in Illinois those who are age eligible for kindergarten are excluded from the PreK programs. The only kids eligible are 3's and 4's prior to Sept 1).
I was beyond thrilled when I found out our school district had the funds to do this transitional kindergarten. Besides the fact that it's free, it meant he didn't have to change schools. His preschool teacher was very encouraging. Our school district screened all incoming kindergartners and actually recommended him for this program (my husband wanted to know if they thought it was a good idea or if it was just me), and a couple of weeks ago his teacher told me she thought it was the right decision and that he might really have struggled in kindergarten.
For me it was really his social maturity (which you can't do anything about, it just takes time) and his size. Also, "No Child Left Behind" has really forced school districts to do more and most are much more academic in kindergarten. Kindergarten is the new first grade. They are really expected to sit still much more than I was, and I just don't see him excelling just yet.
I really feel it was the right decision for us. Hopefully you can talk to people in the school and get some helpful feedback. But by and large, you have to trust your gut.
Good luck!!!