Am I the only one who does not imagine neither Mur nor Liz as old people at all? o.O
At least up to what we know about The Core and all. Sure, Mur spent 32 years exploring the Sub, etc, etc. But it wasn't really explained where those 32 years took place. Probably after the whole Core and Lighthouse stuff. The issue is that we do not know exactly how the chronology goes, so what we've been told(?) in the Lab and the whole thing with people being sent over the Edge and dying might have happened well after the events of, say, Sub8.
Come to think of it, the storyline of Submachine is a bit confusing. We know that, Fourth Dynasty aside, there was the Core, with Mur and Liz, he lost his arm, went to the Lighthouse, got buried, escaped to Subgod-knows-where, and then everything gets kind of foggy. The Exploration Teams were created while Mur was still on the Core, or well after he escaped the burial of his Lighthouse? From then on, what did he do until he started sending people over to the Edge and the Defense Systems? Did Mur spend 32 years exploring the Sub before making the Exploration Teams, or was that note talking about this fact written *after* he finished those 32 years?
...Anyways, I do believe Mur/Liz weren't that old for a good portion of the plot line. They might have well hit over their 60s by all the stuff we knew from The Temple and all (Multi-layered bullets or something, Elizabeth saves him, he does his thing at The Temple, repaired the Layers, being stuck on the Eighth Layer for all eternity of time or something, etc). And that's considering how Liz still carried on Mur's message for many years after his death until it was her turn to pass away.
tl;dr headcanon
EDIT because people are now discussing their thoughts about Mur's looks and all:
I always pictured Mur as a sort of "normal" guy, on his early 30s or so, with shoulder-lenght brown hair - sort of like what James Hetfield had back in the 90s - and a labcoat. Always a labcoat, for some reason.
...Minus the heavy metal singer part.
Liz pretty much looked like Annette Birkin from RE2, but with longer hair. More akin to her concept drawings, not the in-game models - those looked weird, IMO, and she looks better on the concept art.