I think you need another category for 50-75, because 70+ is categorically not middle-aged. Most of the people in my family didn’t even live to 75, and were pretty old in their 60s.
Old codger.
I think of it more as a state of health and state of mind. My Aunt Pat is 85, no way on earth is she “elderly” and anyone who says so best run for their lives because she will catch you and show you exactly how not elderly she is. 