Apuesto a que esto es lo que estás buscando:
SELECT * FROM mytable
WHERE TIMESTAMPDIFF(YEAR, mytable.birthday,'$currentTime') > '$ageFromForm';
Para ordenar los datos, haría esto:
SELECT *, (TIMESTAMPDIFF(YEAR, mytable.birthday, '$currentTime')) AS age
FROM mytable WHERE TIMESTAMPDIFF(YEAR,'$birthday','$currentTime') > '$ageFromForm'
ORDER BY age;
Espero que esto ayude;)eslavo