Find the Yelp user with the average 5-star rating of all reviews who has been yelping the longest.
Yelp user refers to user_id; average 5-star rating refers to user_average_stars = 5; yelping the longest refers to min(user_yelping_since_year)
SELECT user_id FROM Users WHERE user_average_stars = 5 ORDER BY user_yelping_since_year ASC LIMIT 1