This article is from Australia, but I've frequently heard of the same thing happening in the US. A kid is promoted to the next grade, regardless of whether he's ready for it, for social reasons. I think that this does a kid a disservice. I understand their reasoning - they want to keep the kid with his own social group, for his comfort and theirs. But it doesn't work that way in the real world. In almost any jjob, you're going to have a wide range of ages, social groups, etc. It's much more useful for a child to put knowledge into his head, so that he can be successful later in his life.