I'm going to dip my toe into this particular pool apropos of nothing.
I'm an older person (56) who has had a minor bout with cancer that has left me with kidney troubles, no body fat, and a couple of other health issues. I go to the gym 3 times a week. My goals are solely to maintain what little muscle mass I have and maybe show some periodic improvement. As such, my fairly pathetic routine is:
a 25 minute brisk walk though my neighborhood, averaging 115 steps per minute. (For reference, military marching is 120 steps per minute.)
2 sets of 20 pull downs from a sitting position (currently at 80 lbs/36 kg each)
2 sets of 10 pull downs from a standing position (currently at 65 lbs/20 kg each)
7 sets of 20 (plus another 1 set of 10) elevated leg lifts. I don't know the name of the frame you use for this, but you step up and hold your body in position with your forearms and raise your legs in front of you.
8 sets of 25 back arches/lifts on a yoga ball. Essentially I kneel and rest my stomach on the ball. I use 4 basic poses for the exercise: arms outstretched hands clasped (aka Superman flying pose), arms fully straight and forward (aka the "we're not worthy" pose), arms straight to the sides (aka the T-pose for dominance) and finally arms behind the back with hands clasped (aka I don't have a smart-ass name for that one yet).
6 sets of 20 push ups interspaced with 6 sets of 20 squats (do one set of push-ups, wait a minute, do one set of squats, wait a minute, lather, rinse, repeat)
stretches
cool down while complaining internally about how old I'm getting.
When I opt to go for any improvement, I'll increase the weight of a single set of pulls downs by 5 pounds, add another 5 leg lifts, add 1 to each set of back arches or add 5 to the total number of push ups or squats. I generally add one increase (if my body accepts the new terms and conditions) every other week, scattered among the exercises.
So there ya go. If you think your exercise regimen is lacking, you can at least look at this one and say "Wow, at least I'm not doing that badly." /s