To maintain fitness and conditioning from middle age onwards, there are just a few exercises that are most efficient and essential. They are efficient because, combined, they are full-body, all-muscle engagements which require balance and control, and which can eventually pretty much eliminate the need for other exercises. This applies to gals too.
If unfit, it seems that most of these need to be worked up to gradually with smaller muscle groups (eg calves, quads, curls, pull-downs) to be able to handle them with proper technique and without pulling a muscle or damaging a tendon. That's called conditioning.
I mention this topic today because I have just started with barbell squats so I am slowly getting to the full deal as I move from unfit and sort-of weak but fully-functional to borderline-fit for my age. An adventure, physically and mentally:
Push-ups
Pull-ups
Bench press
Barbell squats
Planks of all sorts
Dead lifts
Plus...general fitness interval-type cardio aerobics of a great variety of fun sorts on off-days, from heavy ball smashes and throws to farmer walks to lunge walks to step-ups to squat+mil pushes to ropes to sprints (elliptical, bike, run, swim, row, etc). It doesn't need to be boring but it does need to be stressful and varied. If not stressful to the max, it's a waste of time: if you can move afterwards and are not dripping with sweat it's not tough enough to make much difference. Gasping and heart pounding for 20-30 minutes.
Lots of the exercise machines and lighter weights seem to be made just to raise you to the point that you can do a few sets of those simple, basic, demanding multi-muscle resistance exercises in a reasonable time before you go to work in the morning. For example, I can't do 20 push-ups or 10 pull-ups yet, so we try to condition the things to make that possible. Intensity, simplicity, and weights save time and money. (nb again - this is not about body-building, it's about maintaining general fitness for life for as long as possible)
This is what I am learning. I get lots of arguments from readers. Go ahead and argue with me while I begin to rediscover my 22 year-old abilities. Painful and deeply-fatiguing, but do-able over time. I know a fit 55 year-old gal who can do 35 regular pushups. She shows no muscles and looks fully trim and feminine.
