Forget all that stuff from Dark Empire/2 for a second. What if, say he's just really really old? (and using his Dark Side Face-Dancer powers to appear as "only" a normal man in his 60's)
Other Force users have taken their frail, aged bodies and made them as sprightly as any youthful person (even though Lucas keeps saying that old and injured Jedi are weaker than youthful and able-bodied Jedi). So Palpy takes it one step further by using the Force to appear younger and perhaps even BE younger. A Dark Side fountain of youth so to speak. But even if he doesn't appear young, he goes on living, as an Immortal Sith Emperor, without any need for cloning bodies!!!
This is a half-baked theory I admit, but it might help explain the (canon, as of the ROTS Visual Dictionary) claim that Darth Plagueis REALLY WAS Palpy's master. The VD also claims that the "rebounded Sith lightning" is what "deforms" his face. So his disguise drops when he accidentally hurts himself with another Force power.
And yet Plagueis is also a "legendary" figure. Assuming it's even the same Plagueis of course.

Then again this Plagueis stuff could all be a bunch of baloney cooked up to impress Anakin. But I bet there is already a bio on him in some EU book being written, so this probably won't even matter...