A septuagenarian is a person who is between 70 and 79 years old — someone in their seventies. You can also use it like an adjective: a septuagenarian writer. It is a rather stiff, newspaper-and-biography word. Kind everyday English says he is in his seventies, or she is seventy-four. Writers use septuagenarian when they want to sound exact, or a little fancy. English has a set of these age-words from Latin numbers: sexagenarian (sixties), octogenarian (eighties), nonagenarian (nineties), centenarian (a hundred). They can sound like a label stuck on someone for being old, so use them with care about living people. Though actually, it does not mean “very old” in a vague way, and it does not mean exactly seventy.

七十多歲的人。書面可說年逾古稀（古稀＝七十歲，台灣仍看得到）；口語就說「七十幾歲」。