In the second Mistborn series, Marsh, aka Ironeyes, is still alive. But it explicitly states in the first series that Inquisitors do die of old age. So how is he still alive?
Atium compounding. He uses Feruchemy to store youth in Atium and then Allomancy to burn it, so he gets more youth of it than he put in, granting immortality as long as his Atium supply lasts. Same method as the Lord Ruler, in fact.
The author actually stated this directly in response to a fan question...
And what happened to Marsh? The book doesn't mention him after he fights with Elend.
Marsh is alive. I changed this from when I talked to [Peter]. I realized some things about his use of Allomancy that would allow him to survive. Actually, he is immortal. He can pull off the same Allomancy/Feruchemy trick that the Lord Ruler did. (And he knows it too, since he was there when Sazed explained how it was done in Book One.) He's actually the only living person who actually knows this trick for certain. (Though there's a chance that Spook, Ham and Breeze heard about it from Vin and the others.) So yes, if there were another series, Marsh would make an appearance.
I thought that trick required atium and involved burning the atium. With all the atium gone and Sazed not making any more, it would therefore not be possible even for a full mistborn/feruchemist. Am I wrong, is Sazed providing atium specifically for Marsh to allow a friend and valuable servant to survive, or what?
Marsh has the bag of Atium that KanPaar sent to be sold, as well as several nuggets in his stomach. So, I guess 'immortal' is the wrong phrase. He's got the only remaining atium in the world and can keep himself around for a long, long while—but he WILL eventually run out. Unless Sazed does something.
Note that Marsh has more spikes than the usual Inquisitor, most of them do not have an Atium feruchemical and Atium allomatic spike.