Kindergarten.
Go to www.wrightslaw.com, scroll down the left and click on "Retention" and you will see that holding kids back is a bad thing.
If he is academically on target, you don't have to worry so much about what he could loose if he has a learning problem, but you do have to worry about what will happen to him when he is in High School.
Kids who are older than most of their classmates are more likely to use drugs, drop out, and have contact with the juvinile justice system. THAT is the data you need to think long and hard about before you hold him back. He is old enough, and he is ready, they set cut offs for a reason, don't second guess them. Send him.
Martha
Edit: I meet people every single year in my line of work who not only regret holding thier child back, they are DEVISTATED when they learn that they cause their child such a terrible life long problem. As an educational advocate for special needs kids, I usually meet Mom and Dad when the child is between 14 an 16. Usually, a boy, who was held back because he was needed to "mature." Mom and Dad noticed some speech issues, but were told that they would get better all by themselves. This young man usually starts having trouble in 1st grade with reading, and Mom and Dad say something about it being a problem, and they are told he is not too far behind his peers, who all happen to be one year younger than he is, but the school just wont take that into consideration.
He struggles, through second and third grade, gets a little extra help which the teacher says is sucessful, and they would see it if he only tried a little harder.
He fails the state reading assessment in 3rd grade. They discuss holding him back again, and his parents agree. The next year, when he fails the reading assessment again, it is not quite as bad, so they promote him and give him some extra help, which means that they cut back on what he is learning, and he really only learns half of what every other kid does that year. By this point it really does not matter much what they do, his window of opportuniy to learn reading without great difficulty has long closed. (by age 9) He continues to struggle, and it is all because he is not motivated.
Fast forward to getting his learners permit for driving, he can't sign his name, and he struggled to read the exam on his own, and it took three trys to pass it. His grades have been C's and D's, still a "motivation" issue, because he spends as much time in the principals office as he does the classroom. When the school suggests to this boy that he consider "vocational" training, his parents panic, and call me after the school refuses to test him to see if he is dyslexic (always the condition that is most palitable to parents at first glance.) I cannot tell you how often I see this boy and his parents. Once tested, he has an obvious processing issue and had he just gone to Kindergarten on time, would have been a glaring gap between his level and his same aged peers, and motivation would never have been questioned. By this point, he is reading at a 3.5 year level or worse, he has very little knowledge of science, history, or language at all. His math skills are higher, because he hangs in with that until they hit word problems, and then his "motivation" causes him to fail in that too. His IQ is usually quite good, 110 to 120, but he is now in the fight of his life to learn all that he has missed. It seldom happens at all.
The idea that no child is harmed by being held back is HUGE bunk. It happens all the time.