Spreading discoloration (age spots, etc.) How fast does it change?

[Khoufu]

I'm curious how quickly (or slowly) discoloration such as age spots, regrind, acne cream damage, and others may spread. Photos are most welcome if you have "then vs now" examples.

[lovesbabysquirmy]

It totally depends on 3 factors:  the quality of the original batch lot of plastic when the pony was made, the conditions under which it was stored, and any kind of treatments that they may have received in owners' hands.

So.... it's very difficult to predict!