There is no requirement to do GCSE music prior to A-level music (and you may find that your aural and theory skills have been met already) nor is there a minimum age for the A-level. The trouble you will have is studying the A-level unless your school is willing to accommodate you.
It's a difficult A-level to self teach because of the coursework components. Private centers may not be able to offer the A-level because of the lack of facilities and teachers with the relevant expertise.
If you do manage to successfully organise teaching, I'd still recommend at least getting a GCSE Music summary book so that you can brush up on the gaps in knowledge that you may have.
I'm currently in year 11 with grade 8 violin and grade 6 theory, doing GCSE music. GCSE Music is actually much harder than it seems, mostly because what I write is too complicated or not in the mark scheme etc... so I would recommend doing it because it will get you used to the basics of how those kinds of exams work.
Then again, that's really mostly exam technique which you will learn at A level. I agree with Kvothe that you might want to get a summary book for GCSE to learn some of the things you might not know. It might not be possible to do it at school because there's more teaching hours which can be hard to timetable, but if they do let you or if you can find a centre to do it at, then I don't see any other reasons why you shouldn't!
Good luck with whatever you choose x