That's really common. Plastic yellows with age as it deteriorates. It happens faster if improperly stored. A hot, stuffy attic isn't good, but sunlight is the absolute worst for discoloring plastic. It's very normal for vintage figures to be discolored, especially the white ones. And since the bodies are often made from a slightly different plastic than the limbs, the amount of discoloration will vary. As for the newer stuff, Hasbro has had some issues with plastic of some figures being less stable. There's a bad batch of clone troopers that can currently be found at retail that all have yellow helmets already, & they're brand new for sale still in the package. Hasbro did a second run that has better helmets (so far) but many of the yellowed ones can still be found at retail across the country.