To answer the above question, the relevant sentence is: 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.
This justifies what answer for what question? Q & A: 
Passage: 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.

Question: Based on this passage, how old are you when you finish sixth form?
typically between 16 and 18 years of age