But they have nothing to do with emissions & when they break there are no more emissions.
huh? cam timing can be off due to belts, such as a stretched belt, which affects emissions. some engines are not interference engines, jumping a tooth on a belt, engine would still run, but bad emissions.
the materials of the belt do not degrade, otherwise Honda would have given a time to change if age was a problem on their flagship model which could be had with a seven yr warranty.
the belts are very under stressed, not exposed to the elements and high underhood engine heat.
I just checked mine at 62000 miles when I was changing heads. The belts looked new, zero wear.