septuagenarian UK /ˌsɛptjʊəd͡ʒɪˈnɛəɹɪən/US /ˌsɛptəd͡ʒɪˈnɛɹiən/adjective 1noun 1forms 1 adjective1 •Of or related to seventysomethings. noun1 •Synonym of seventysomething: a person between 70 and 79 years old.Mary Moon, she's a vegetarian. Mary Moon, Mary Moon, Mary Moon. Mary Moon, who will outlive all the septuagenarians. — 1994 formsseptuagenarians