second childhood

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. Senility; dotage.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. The period or state of cognitive decline of an elderly person, characterized by childlike judgment and behavior.
• n. A childlike state in any adult, resulting from mental illness, trauma, or other conditions.

from the GNU version of the Collaborative International Dictionary of English

• n. the state of being feeble and incapable from old age.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. mental infirmity as a consequence of old age; sometimes shown by foolish infatuations