in one's dotage vs septuagenarian

in one's dotage

septuagenarian

adj

  • Of or related to seventysomethings. 

noun

  • Synonym of seventysomething: a person between 70 and 79 years old. 

How often have the words in one's dotage and septuagenarian occurred in a corpus of books? (source: Google Ngram Viewer )