God, they were already talking about brittle plastic in 2004!
It's been going on for ages. As I'm sure you know, the PowerBook 190 and 5300 were notorious for brittle plastics (particularly in their hinges)
when they were new, so age need not be a factor in every situation (it certainly wasn't for the 190/5300!), though it does seem that plastics that were cheaply made/marginal when new (ex., the aforementioned 190 and 5300) have aged particularly badly.
This includes most Mac laptops and many desktops and towers manufactured from 1992-ish to 1997-ish.
The so-called SpindlerPlastic formulations (so named since much of it seemingly entered production when Michael Spindler was CEO) seem to have lasted as late as 1998 in some supply chains (the last of the beige hardware, pretty much); it seemed to largely resolve after Apple transitioned to using more metal and polycarbonate-type plastics (a notable exception to this is the inner ABS framework on most slot loading iMacs, which are terribly prone to shattering). However, while polycarbonate doesn't seem to get terribly brittle with age, it has a most unfortunate tendency to crack (notable examples of this include B&W G3 handles and feet and clamshell iBook display bezels, which
always seem to crack around that little inset Apple logo that sits below the display)
.
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