Conversation so far:
When did John Lennon meet Cynthia Lennon?
Lennon met Cynthia Powell (1939-2015) in 1957.
Did they meet while attending school?
in 1957 when they were fellow students at the Liverpool College of Art.
How old were they when they met?
CANNOTANSWER.

Question: How long before they married?
Self-contained rewrite: How long before John Lennon and Cynthia Powell married?