It’s just as important to ask why a Pope would want to resign, as there are several possibly good reasons reasons if not other than, but in addition to age and health.
One of the errors of many great leaders is that they neither prepare, appoint wisely, or have any involvement in choosing their successor. But having a mediocre or poor successor can, in a manner of speech, “snatch defeat from the jaws of victory”.
Thus, retirement might have in mind the appointment of a successor who will offer the position more vitality and stamina when he is younger, and stability in the appointment when stability is important.
Pope Benedict has named many, if not most, of the Cardinals who will select his successor.