MySQL 5.7

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CREATE TABLE tablename(birthdate VARCHAR(20));
                                         
INSERT INTO tablename(birthdate) VALUES
('1950-05-15'), ('1960-08-00'), ('1970-00-00');
SELECT birthdate,
       YEAR(NOW()) - birthdate AS age
FROM tablename


birthdate
age


1950-05-15
70

1960-08-00
60

1970-00-00
50


SELECT birthdate,
       COALESCE(
         TIMESTAMPDIFF(YEAR, birthdate, NOW()),
         YEAR(NOW()) - birthdate
       ) age  
FROM tablename


birthdate
age


1950-05-15
70

1960-08-00
60

1970-00-00
50