Having a late birthday i was actually only 17 when i took the exams, though 18 when i received my results, i have read this is rare for Grammar school students.
A friend of mine only just qualified for my year having a mid August birthday so was only 17 on results day, and actually still 17 only about 10 days before the start of the next academic year.
Two of my year actually stayed an additional year (3rd year 6th or year 14) to try and beef up their results, it was not common but most years at least one person did, not sure if this practice still continues (i read that if you are 18 or under on 1st August you can remain in school or 6th form college).
Also back in the 60s apparently Oxbridge candidates did an additional term at school??? It is actually mentioned in the obituary of a teacher who died suddenly and was also a former pupil, no idea how this worked.