Mine was relatively late too, picked up my stammer from when I was 16 which didn't help my French and English GCSEs at all. Developed over AS and A2 when it was at its worst I would say. I found that a 'cure' doesn't really develop but the way you manage your stammer definitely changes. From the way you substitute words subconsciously and say certain words as well as your confidence definitely improves. I'm 24 now and I'm so glad I'm not the same way I was when I was 16-18. I am resigned to the fact my stammer will never go away though.