Photodegradation, more commonly referred to by toy collectors as "yellowing," is the phenomenon by which plastics discolor over time. The process is generally started by exposure to ultraviolet light[1], whether from natural sunlight or fluorescent lighting. Photodegradation is most noticeable on figures with bright white or light grey plastics, although it actually happens to virtually all colors. Regardless, once the process is underway, it is impossible to reverse.
In addition to there being no way to undo photodegradation, there is also no hard and fast way to determine which individual figures will be more susceptible to it. A Jetfire opened once, then left in a closet for 20 years seems just as likely to be affected as one left sitting on a shelf for the same amount of time. Furthermore, different pieces of identically-colored plastic on the same toy can photodegrade differently, resulting in a Nightbeat with a normally colored canopy and discolored legs. Even more significantly, areas covered by decals are usually unaffected by the process.
Photodegradation is not to be confused with cigarette smoke residue, which also leaves a yellowish cast on plastic (and can be removed with toothpaste or other products designed to remove cigarette stains) or sun damage, a bleaching effect caused by exposure to direct sunlight (which more commonly affects packaging materials).
The Transformers fandom has struggled with this issue for ages, and despite having the answer for some time, fans still often tinker with various concoctions hoping to reverse this scourge. Simply put, if a liberal amount of denture cream/household cleanser/virgin blood gets the toy white again, it wasn't photodegradation, it was your common everyday grime that you removed. Some people have experimented with physically removing layers of plastic from toys (via fine-grit sandpaper) to remove photodegradation, but this seems like a temporary fix at best (and one that actually may damage the toy more). Unfortunately, the odds of reversing the photodegradation process are about as likely as turning a hamburger back into a pile of raw meat. Toys are not designed to last forever, and engineers in 1984 most likely were not taking into consideration how well the plastics would hold up 25+ years later.