Have you read The Nurture Assumption? There's a chapter on that; in the West someone who's small/immature for his class level will be at the bottom of the pecking group throughout his education, whereas in a traditional society where kids self-segregate by age in a more flexible manner, kids will grow from being the smallest of their group to the largest of their group, so will have a wider diversity of experience.
It's a pretty convincing reason to not make your kid skip a class.
Also a good reason to consider home-schooling or even having them enrol in primary school education one year later.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.