if you want to be able to briskly climb stairs when you are 75, you need to be in the top 95th percentile of cardiovascular fitness. Even at 95th percentile, it’ll be hard to jog up steep hills, etc — you’d have to be an elite athlete.
But if you are average/low, you may not even be able to do any of that. And it seems unlikely you’ll be avg/low now (I’m in my early 40s) and then somehow go from 50th percentile to 95th. So basically it’s better to get started
Fun graph from Peter Attia’s book Outlive