second childhood

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Senility; dotage.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

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

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