For example, squatting preserves the ability to go up and down the stairs because it mimics the movement. Advanced variations (or adding load) is necessary, though. Otherwise anyone just going up and down the stairs a couple of times a day would manage to do it.
Carries preserve the ability to carry your groceries. Think farmers walk and similar. Any isometric carrying of a higher load helps. The higher the load you can carry in 1 minutes, the higher the load you can carry as shopping for several minutes.
These are 2 examples. Think about the movements you are going to need when you're old, and what lifting/calisthenics/gymnastics/strongman exercises relate to it, and do it 2-3x a week, with normal growth principles (like progressive overload) in mind.
If you want to be able to run when you're old, you need to run. That's the easiest to figure out. Run long for stamina, fast for speed
This is not just for aging. Anyone who doesn't do something, stops being able to do it. For example, many unfit people are unable to run for 5 minutes, some not even for 1 minute. Someone who always take the car to go shopping always struggles to their bags, even from the shop to the car.
If you don't put load on your shoulders (i.e. sandbags or barbells or even a backpack with increasingly heavy staff inside) and walk with it, you will struggle to carry your kids and your grandkids. You may even get back issues, which all lazy people past a certain age though.
It's not just about strengthening certain muscles (although that's necessary apart from what I'm saying) but also about replicating movements. Replicating movements engages your nervous system and develops neuromuscular coordination, making the movements easier. This can also be observed when learning something new: the demand for your brain is much higher when learning something which can leave you worn out and increase the intensity of resources used by your body; but once you learned it, your body and brain know how to do it, so it takes less resources (including energy and effort) to do.
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