SQL & PL/SQL :: Trigger From Multiple Tables

I need to set up a trigger to check and enforce that the age of an employee at hire date is older than 18 when a new record is inserted. The age is on the PERSON table, and the hire date is on the EMPLOYEE table.

My attempt:

CREATE TRIGGER AGE_HIRE
BEFORE INSERT OR UPDATE ON EMPLOYEE
FOR EACH ROW
BEGIN
IF FLOOR EXTRACT (YEAR FROM (PERSON.BIRTHDATE - EMPLOYEE.HIREDATE)) > 18
THEN DROP NEW
ELSE INSERT NEW
FROM PERSON, EMPLOYEE
END;