Informix trigger to delete records before insert
If you follow the syntax diagram(s) for CREATE TRIGGER, you find that the action clause needs either BEFORE, FOR EACH ROW or AFTER as a prefix before the action verbiage, and the triggered action segment shows you need parentheses around the triggered action(s).  Hence:

CREATE TRIGGER systables_growth_clnup
    INSERT ON systables_growth
    BEFORE (DELETE FROM systables_growth
            WHERE date_added &lt; (CURRENT - 15 UNITS DAY)
           );