Does this line have to be defined strictly by hardware, or is software a factor?
Maybe it's just me, but when I think of what defines an "Old PC" it's DOS, and arguably the last (mainstream) gasp of "DOS-based" operating systems was Windows ME, released in September 2000 and obsoleted by Windows XP in October 2001. (For the purposes of this discussion we'll ignore Windows 2000, which was *arguably* the first "Consumer" version of NT but wasn't quite for everyone.) Therefore to my mind the outer limits of what qualifies as "vintage discussion" would be Windows ME-era hardware *running similar vintage OSes*. The second part seems important to me; a thread about building the ULTIMATE Windows 98 gaming box around a first-gen Willamette Pentium 4 might qualify as a trip down memory lane but a thread about the same hardware running Linux or Windows XP or whatever? Meh. A system like this running a modern OS (and I'm lumping XP into "modern" here, as comical as that might be) is a "Tweener" or just "old", not "Vintage".
Another thing to think about: is it really correct to lump a range that starts with Socket 4 and goes all the way into Coppermine territory into a single "Pentium" forum? Windows 95 came out in the middle of the Classic Pentium era so in a number of respects an early Socket 5 box that shipped with DOS has more in common with a 486 than it does with a late-model Socket 7 running Windows 98 even though it's technically the same processor generation. To keep it in three categories ("PCs and Clones", "Older PCs", and "whatever but still old") I'd probably widen the "Older PC" category to say "Later MS-DOS computers, 386 to Early Pentium" whilst saving the last category for "Win 9x Era" machines, however they're described.
Windows ME and the Pentium 4 debuted within a few months of each other so setting the hardware cutoff for the last category as "Anything less than a Pentium 4" is probably close enough.