In the education systems of England, Northern Ireland, Wales and some other Commonwealth countries, sixth form (sometimes referred to as Key Stage 5) represents the final 1 - 3 years of secondary education (high school), where students (typically between 16 and 18 years of age) prepare for their A-level (or equivalent) examinations.
how old are you when you finish sixth form
(typically between 16 and 18 years of age)