Unfinished Tales tells us that in the early days in Aman, Olórin "walked among [the Elves] unseen, or as one of them." He did not take the form of an old man in his 'youth' in Aman. This incarnation was imposed as a condition of his mission in Middle-earth. The incarnate form brought with it all the hardships of the flesh, including hunger, pain, weariness, and more. He was no doubt exhausted when he returned to Aman, and I think it is safe to assume he abandoned the incarnate form as soon as he was told that his mission was complete.
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