SELECT COUNT(user_id) AS "Number of Users",
           YEAR(CURDATE()) -
           YEAR(STR_TO_DATE(birth_date, '%m/%d/%Y')) -
           (RIGHT(CURDATE(), 5) < RIGHT(STR_TO_DATE(birth_date, '%m/%d/%Y'), 5))
             AS Age
    FROM user
GROUP BY Age

SELECT Age,COUNT(user_id) AS "Number of Users"

FROM
(
    SELECT userid,
           YEAR(CURDATE()) -
           YEAR(STR_TO_DATE(birth_date, '%m/%d/%Y')) -
           (RIGHT(CURDATE(), 5) < RIGHT(STR_TO_DATE(birth_date, '%m/%d/%Y'), 5))
             AS Age
    FROM user
) as Z
GROUP BY Age