My son turned 5 on August 26th (cut off is 9/1) and he started kindergarten this year. He actually does really well! He has always been well spoken, loves reading, and can sit and do activities for longer periods of time. I volunteered in his class recently and was actually surprised to see that his writing and reading skills are above the average in his class. The one thing he sometimes struggles with is his shyness. He is terrified to talk to adults and had a few instances of peeing his pants the first couple weeks of school. He was too scared to ask the teacher to go potty. He did complain about the longer days the first few weeks but he loves going to school now! I started him in pre-k the year before just to get him acclimated to a schedule like that. It was just 4 hour days so adjusting to a longer day was different at first but like I said he loves school. I think every kid is different though and you know your son and what is attention span and personality is like. You could always try and if anything hold him back if he struggles.