I really don't think that this is a good idea. It moves us even further towards the notion of exam grades as a 'sorting' function, rating people from top (clever) to bottom (to be shuffled off into some 'non-academic' hole).
The point of GCSEs was meant to be that they were criterion based, identifying whether you had learnt a certain set of skills and knowledge.
Frankly, as an employer, I don't care whether you got a C grade at GCSE at age 10 or 20, I care whether you can write a coherent letter and work out percentages correctly every time. And indeed, under this suggestion, how would mature students' exams be marked?
Its a bit like a driving license - you have to achieve a certain level in order to be able to drive. We don't say 'you're not a natural driver, so stay on your bike' - we say 'you need to take more lessons and work harder to achieve the required level.
I absolutely agree that summer born babies need more support. As others have said, why not just relax the absolute rules that say 'you must stay in your age cohort entirely' and allow teachers and parents together to make a decision as to when it is appropriate to move a child out of year.
DD's year has exactly the same issue at the other end; the two oldest children in her class - who are also quick learners - should patently be in secondary school now, not kicking their heels in primary for another year getting into trouble because they're bored.