Dunno. At 11 and without all that much practice I was finding it not difficult to do standard KS3 matsh SATs (now abolished), but never really bothered to go faster. As a result I basically spent five years of maths lessons learning not all that much and just being bored, walking out at the end of that with almost full marks in the GCSE. Maybe I could have sat that earlier and with similar results, but I think it was effectively decided that I not do so and not "rush" exams. But I think I could have sat the GCSE, and maybe even the A-level with a bit more work, some years earlier than I did.
Thing is, with all respect to those who have sat it, the Maths GCSE at least isn't all that hard. (As far as I'm concerned exams have got easier over time, though probably not year-on-year.) Anyone who has a "maths brain", whatever that means, should find it a breeze. Beyond GCSE, A-Level is definitely a step up, but what the "maths brain" means, presumably, is an ability to follow a logical path and spot patterns, which is essentially the entire subject of maths. It's not that there is only one right answer (sometimes there isn't an answer at all), but rather that you are "forced" logically to get to any answer there is, and by that same logic can identify all the wrong answers. GCSE and A-Level papers, which often split the problem up so that the logical route can be taken in small steps, therefore play into the hands of people whose minds work that way. Hence why particularly gifted young children can sit such papers at a young age. Unlike any other subject, maths tells you what the next stage is, and the only difficulty (although it is of course extremely difficult at times!) is in being able to read those signposts.