It can be a lot easier on the kid to start just a little later than a little earlier. If he is gifted it won't matter since he will be learning on his own no matter where they are in class, but he will fare better socially if he is somewhat older rather than one of the youngest all the way through.
Interestingly, I was listening to a biographical tape of a Russian immigrant who stated that grammar school started at age 7. This was a thoroughly Westernized education, excepting some religious classes. And in universities back then, the only test (at least that I know of in the humanities) was an oral examination at the end of the year. This school system produced some of the world's greatest writers and composers so it must have been quite good.