I think it's Enoch Root that really pushes it into sci-fi territory. He's also a major character in The Baroque Cycle, implying that he's several hundred (or more) years old.

Well, there is Enoch Root, who I have never been entirely clear on. I haven't read the other books set in that universe however.

I believe the name "Enoch" is a hint though. Enoch in the Book of Genesis is the father of Methuselah and importantly did not die (but was instead "taken up", which admittedly is not much more clear than Cryptonomicon...)