I don't really think Athlon XP is retro. Mainly because I built a few Athlon XP and Pentium 3 machines for various friends and family and some of them are still in use - all running Windows XP.
I believe they are obsolete but not really retro.
I kinda feel like Socket 3 and earlier is 'retro' but anything newer is merely obsolete, because Socket 3 tends to be radically different from any machine you see today. Socket 5/7 (and even some late Socket 3 boards) blurs the line a bit but anything after that feels a bit too recent for me - once you start getting ATX boards that use PCI and DIMMs, with onboard IDE, sound, network, video etc, and don't support external cache on the motherboard, you start getting too modern for my tastes. Most of this stuff is considered obsolete but there's still plenty of systems out there that use it - and most kids these days that tinker with computers will be able to at least recognize most of the parts and repair such a machine. Give them a VLB 486 though and they'd have no idea.
Of course its all personal preference. I've had discussions with people that think anything from the 90's is too recent to be retro.
If you are squeamish, don't prod the beach rubble.