Your description of the "always old" actor made me giggle and also think of David Attenborough whom I swear was old when I started watching his documentaries and now I'm quite convinced that he's actually a fae.
Some people just have great health (either through luck of genes or taking care of themselves or some combination thereof), and the difference can be staggering in old people. Like I see some scuttling about in ways that would put 20yos to shame (my parents are both working in reasonably active jobs, though they like to pull out the "getting old" card every so often, they are also the "yeh gonna retire at 60...no 65..." still going in their 70s) and others that just don't do very much at all because "too old" in stark contrast to the former.
I hope I'm in that great shape when I'm 70 XD
I think it's good that some of them can keep going anyway, it's always helpful being able to represent all types XD
I'm genuinely impressed at how Steve Martin is 78 and in the show he's running around. But yeah he really is an actor I've been convinced has always been old. He barely looks any different from the 80s. A few more wrinkles but that seems to be it. Martin Short also looks pretty good for his 70s.
David Attenborough is something else. He looks his age but just refuses to go. He's the kind that'll end up as a floating head in a jar like in Futurama.
Bahahahhahahahahahhaaa XD Well...not wrong? XD
the idea of that kind of existence horrifies me x_x