Found the answer to my question from posts on another forum:
"The afterglow of Superluminova paint is not due to radiating its internal energy. It is reradiating energy it "captures" from incident radiation (visible light). The process is called photoluminescence. This is why one has to "charge" it. There is an internal process that delays part of the reradiation and this is the "afterglow".
Also, I believe the"aging" to which Dave refers is the phenomenon of color change/patina/etc. which is not the exact thing as the lack of glow, although it is somewhat related.
The old tritium-doped zinc sulfide paints age, in part, because the radioactive "byproducts" damage the zinc sulfide crystals. This acts, via color centers, to change the color of the paint, as well as to reduce the efficiency of the fluorescence process. The fact that as radionuclides decay there are less radionuclides left to decay is a separate "aging" phenomenon that can also contribute to paint no longer glowing (but not aging in the sense of developing a patina). These paints glow via fluorescence (induced via beta decay of nuclide) not via photoluminescence.
Moisture damage is yet another cause of "aging". Also, what happens with the bonding agent used to keep the paint crystals all in one spot is another item than ages via other phenomena."
"Coincidentally, I work for those guys as a watchmaker (swatch group sav, omega among others), and as an speedy pro owner myself, and WIS, that's one of my points of interest.
any paint, does age, due to damage by (light, UV) radiation, the glowie one being no exception.
UV is another sort of radiation that eventually, gets the paint to age, not only glow paint (tritium, lumi...) but even "non-glow" paint too, white dials become creamy (remember rolex explorer II) bezels fade (on speedy's and seas, particularly on blue speedys) and some dials change colour, on speedy's and flightmasters, the black zones become brownish, and on some lacquered dial seamasters, the blue lacquer fades to reveal the metal dial, leaving all the writing and wave pattern, intact."
An interesting link:
LUMINOSITY IN WATCHES