As a teacher I would say that anything is possible. A couple of years ago I taught a boy who was pretty 'average' all the way through school, L4 in primary, level 5 KS3, Bs, Cs, Ds at GCSE, nothing to set him apart really. Absolutely flew at A-level , got ABB and went on to do a chemistry degree. Something just clicked at the age of 17. It was very lucky I'd actually taken him onto my A-level course (chemistry) as he didn't actually meet the entry requirements, glad I took the risk!