I always assumed that any volatile components in the plastic in the kits would migrate away over time. Old kit parts always seem a bit delicate and brittle, even if they have been left in the box. UV or even just the bit of extra warmth from sunlight probably accellerates things. I have always used Humbrol enamel, and the painted bits seem to last well. I assume that keeping the styrene separate from air helps.
I have a Lindberg trawler that will be having its 21st next year. The deck is sprung into the hull, and the hull shows no signs of brittleness, but it is rather thick material. Some of the finer details have suffered.
I have never been in favour of using superglue on styrene, preferring MEK or polystyrene cement out of the tube. I just cant bring myself to trust a joint that consists of styrene, a mechanical surface join to another type of plastic (the superglue) and another join back to styrene, when a styrene to styrene weld is possible.