I’m a greybeard, my first OS was either DOS or some OtherDOS, my first GUI environment was System 7 (Mac). The latter was mostly flat other than some dithered shading mostly on icons, significantly flatter than macOS is today. Even with MacOS 8 which gained more depth in its design I always found Windows’ hard bevels through 2k/ME overstated, and when I did use Windows I’d do my best to configure it to be as flat as possible. So yeah I think early GUI exposure might be a contributing factor, probably moreso than age.
System 7 added depth to things like title bars, and then there were extensions like Greg’s Buttons that went all in for apps that used the default controls. With system 7, finder icons went seriously 3D because it was finally allowed to have multiple icons for different color depths. The use of color or even grey was one place where System 4.2-6 had fallen behind Windows 3.0, even though the graphics hardware was more advanced.
You're probably on to something with that. My tastes are such that I'd love a System 6 version of this project. 7 always felt wrong, like they were forcing something the hardware wasn't ready to do yet.
