What follows is me repeating things I heard or found on the internet.
Some lenses turn yellow because of Thorium, which was used in their construction. It's a radioactive element and the yellow tint is the product of Thorium's decay. Or something of that sort. Apparently, or allegedly, this can be helped by leaving the glass exposed to UV lights (sunlight).
Leicaflex' prism becomes yellow due to "desilvering" and the only help is to have the prism replaced. I don't know if it's the same process that happens to old mirrors. Either way, no sunbathing will help.
edit: It's not (only) plastic lenses. 35/1.4 Nikkor (of NC Auto kind) often has a yellow tint, and its glass is real glass. Of very good radioactive quality.