Epson Enhanced Matte contains optical brightener agents (OBAs). OBAs provide a whiter (blue-whiter) paper surface appearance, but over time they will fade gradually. This means that over time, the paper surface will return to its "original" non-OBA color, which is typically more yellowish. For example, if you compare two similar papers such as Moab Entrada Fine Art Natural (which does not contain OBAs) vs. Moab Entrada Fine Art Bright White (which does contain OBAs), you will notice that the Bright White version has a whiter (colder) paper appearance, whereas the Natural version has a yellower (warmer) appearance.
Keep in mind that the OBAs fading is a different process than the inks themselves fading. Both will happen, but at a different rate and not necessarily at the same time.
Nobody knows for sure how long it will be before the OBAs fade, because the papers haven't been out there for that long (if someone wants to volunteer to report back in 50 years or so, that'd be great!).
If you store the prints in the dark, they will fade less quickly than if you display them on the wall in bright sunlight with no glass. No surprises here.
Many fine papers have OBAs. And many people around here have used Epson Enhanced Matte with good results. It's a very economical paper, too. It can be used both for the final print as well as for "proofing" tests before making final prints on more pricey papers.
If you are worried about OBAs and want to go with papers that don't have them, there are plenty of OBA-free ones, but I doubt you'll find any as economical as Epson Enhanced Matte.
p.s. By the way, Epson is in the middle of relabeling their papers. Enhanced Matte is now being called Premium Matte. Whatever the name, it appears to be the same paper.