infirmity vs second childhood

infirmity

noun
  • feebleness, frailty or ailment, especially due to old age. 

  • a moral weakness or defect 

second childhood

noun
  • The period or state of cognitive decline of an elderly person, characterized by childlike judgment and behavior. 

  • A childlike state in any adult, resulting from mental illness, trauma, or other conditions. 

How often have the words infirmity and second childhood occurred in a corpus of books? (source: Google Ngram Viewer )