I don't really have a cut off point and I find different things interesting from year to year. I'd agree though that pre 1997 is a good benchmark for 'retro'.
Also my 1997 Bravado just happened to be the bike I mainly rode between 1997 and 2005, when I finally decided to invest in something more modern.
Nowadays I do however like to keep things more or less period correct. Whether building a bike from the early nineties or a bike from say 2005. That way I think I'm happier comparing the differences in certain developments and characteristics.
I'm not really keen on 'retro-mods' or 'neo-retro' even though I've built a couple for practicality.
I'm slowly working on 'a bike from each year of the nineties'. I do keep getting drawn back to 1992 though...
1992 was the first year I had my first 'proper' mountain bike.
On reflection, and along with my personal history, my favourite years are 1992 for pre-compact drive and the dawn of XTR, and 1997 for V-brakes.
So in conclusion; 1992, 1997 and 2005 are significant benchmarks for me personally.