Orgaz did warn Mevron regarding this matter the first time he learned of Mevron's interest in obtaining the Fruit of Youth. But at that time, his advice was brutally turned down, and because of that, he never brought it up again.
However, now that even Mevron's father was directly involved with the possibility of the Winter Dragon King getting involved, Orgaz had no choice but to make sure Mevron understood the gravity of the situation.