delimiter //
CREATE TRIGGER person_bi BEFORE INSERT
ON person
FOR EACH ROW
IF NEW.age < 18 THEN
SIGNAL SQLSTATE '50001' SET MESSAGE_TEXT = 'Person must be older than 18.';
END IF; //
delimiter ;

delimiter //
CREATE TRIGGER person_bu BEFORE UPDATE
ON person
FOR EACH ROW
IF NEW.age < 18 THEN
SIGNAL SQLSTATE '50002' SET MESSAGE_TEXT = 'Person must be older than 18.';
END IF; //
delimiter ;

delimiter //
CREATE TRIGGER person_ai AFTER INSERT
ON person
FOR EACH ROW
UPDATE average_age SET average = (SELECT AVG(age) FROM person); //
delimiter ;