Given my retro build i started last year is a 2005 build i certainly consider it retro. That said it was a turning point so some of these builds are more retro than others.
To me to qualify as retro the following needs to be the case :
- Contains a BIOS with no UEFI update being availible to modernize the system (This seperates my retro PC from my previous now 10 year old PC that has everything you need for the modern era).
- It has or even relies on obsolete tech, such as AGP, north bridges, old DDR standards, lack of USB3, AC97, parralel ports, IDE, floppy controllers, etc.
- Most importantly of all it can be used to do things modern systems can no longer do. Such as running Windows 98 stable on bare metal will all drivers installed correctly. XP is tricky here because XP still has unofficial support keeping it alive on modern tech but i know that my modern PC would never be able to run it. So in my opinion XP counts on this one, 7 does not since modern PC's can still run it and it doesn't rely on older tech to function. But also things like soundblaster emulation and native EAX come in play here.
- It can be used with Retro parts such as PCI hardware, AGP video cards, IDE hardware and floppy drives. None of which my modern PC can do.
- (Optional but a good indicator) Can not run the latest version of Windows properly.
So i don't think there is a certain cutoff point in terms of year. After all my 2005 Athlon 64 X2 build can almost flawlessly run 98, runs most dos games i tried and still has AGP graphics, ide controllers, a dvd drive with a headphone jack done in a case that feels undoubtedly retro. Yes, its clearly more modern than era's before it and resembles what we have now with the lack of jumpers on the motherboard or support for USB2 . And yes i could stick Windows 10 on it and usably use it as an office PC. But a system reliant on so much old tech to the point the vast majority of components in it are tech we no longer see in the modern day make it undoubtedly retro.