Students can tell who's smarter and who's not, and someone at the same level of learning will always be far more of a peer than someone who happens to be the same age. All the available evidence is that the best thing for children's social life is to let them interact with people at the same learning level rather than the same age. (To say nothing of the fact that segregating students by age is unnatural in the first place, and traditional cultures where kids grow up around people of a wide range of ages are far better for their social development than the western school model...).