Maybe we need a mid-school forum.
Not such a mad idea. There was certainly a period around 98-2003/4 where higher end bikes started to deviate from designs and dimensions the ha gone before, but had still yet to morph into what is now the typical mountain bike today.
Indeed, the era of quality 26" machines is coming to a close so some time around 2013/14 will be the boundary of yet another generation of machines.
We are watching history take place chaps and helping to keep it alive.
Vintage, year dot to 83 or so.
Veteran, 84 to 98.
Transitional, 99 to 2004.
Modern, 2005 to date.
Post modern , the next year or so....
Ok, so flexibility in the actual dates but as a rough guide those are the 4 distinct generations of mtbs that have been so far. I do think just retro and modern is a bit too simplistic when trying to describe the distinct phases of development and history of the mtb.