I really don't buy into the "sunlight" theory....
Toys from my childhood that I have kept in a room with plenty of windows are still white as ever....
However, toys that I stored in a dark but HOT attic with 100% no light exposure are bright yellowish-tan.
I think it's heat, not sunlight, that is the main contributor to yellowing of plastic.
The only thing I've seen sunlight do to my toys is fade the bright colors! (and this is all over a 20 yr+ timespan in the very bright, very hot state of Texas)