Middle-aged and older people suffer from the heartbreak of Apoptosis. In this case, age-related demise of muscle cells. Only strength-building can help us stay ahead of the curve of physical decay as long as possible.
So we're talking only about strength-building exercises here: Squats, Presses (ie bench and overhead), and Deadlifts: Why Gyms Don't Teach the Only Exercises You Need
Rippetoe makes the essential point, but most people would add pull-ups/chin-ups, dumbbell rows, and maybe dips to complete the list. That's enough to keep anybody busy for 2-3 hrs/week, since none of these heavy things should pushed to the limit more than once/week by the ordinary person over 40.
What about "curls for the girls"? Non-essential. Optional, for vanity (ie to impress the girls).
What's on your list of essentials for strength-building and/or strength-maintenance?