Personal Thoughts
There are wildly differing contemporary interpretations and usages of the word 'retro' flying around - it's here that most of our problems relating to this issue stem.
For example, it's probably fair to say that the MTBR (American) view of the word 'retro' refers to the retrofitting of aftermarket parts onto a stock bike, and therefore is often deemed to be a negative activity over a stock build-up (as seen in their often confused view of our BoTM). However it was an essential part of our period culture, regardless of the fact that most of that US-designed and manufactured bling was shockingly bad and expensive!
In the UK most of us here utilise the word to describe a nostalgic period in the European development of mountain biking as an activity, sport and business. This usage has been further tacitly refined through the early days of the site but had yet to be clarified into a date - as it is necessarily a subject of, fairly futile, debate.
Remember that 'retro' has entered the broader public consciousness through things like 'retrogaming'; Eightis parties and even retro cars - where magazines extol the virtues of taking a classic car and applying modern race engines and suspension. This used to be customising or hotrodding but has morphed into something peculiarly British. This wider usage is not what retrobiking is within the context of this forum - in this sense, it is not a broad church, nor should it be. But as the site gets more registrations inevitably drifts towards a 'younger' set of memories. The moderators are right to sharpen the focus of this site and this iterative process may be the cause of friction and flashpoints. These are sadly inevitable but isolated cases, but the buck has to be stopped by somebody!
So a date has been deemed necessary - as I see it, it's purely a functional tool and not worth splitting hairs about (unless you're so inclined). My understanding then is that all frames manufactured up to (and including) 1997 can be considered 'retro' here and are expected to have, wherever possible, period parts. This is what I have always expected when checking in - it's just never been put down in black and white before. Remember, if you are a fan of, for example, retro DH machines, then why not start a dedicated forum? Take this site as an inspiration and develop it up - it would be fantastic to see.
Mr K