Thou shouldst not have been old till thou hadst been wise.
Lear wanted to take all of the benefits from being a king without having to do anything. Lear did not see what would happen when giving his kingdom up to Regan and Goneril and because of this blindness he portrays, the fool foreshadows that because the King was not wise, he should not have given up and "retired" so early. Because he did, he will now lose everything.