What is the customer ID who is associated with the latest policy?
SELECT t2.customer_ID FROM policies AS t1 JOIN customers AS t2 ON t1.customer_id   =   t2.customer_id WHERE t1.start_date   =    ( SELECT max ( start_date )  FROM policies ) 
latest policy refers to MAX(start_date) in policies; customer ID associated with policy refers to joining tables policies and customers on customer_id;