There are some good comments above but I tend to agree with this:
I’ve always thought we mis-used the term ‘retro’ a bit, we normally use it to describe ‘old’ or ‘old-skool’ (for the kids ) but it’s actual meaning is something that imitates a design/concept from the past, or relating to the past, not necessarily being from the past, if that makes sense?
…but I digress, to answer your original query, there is a bit of a grey area amongst the retrobikers, some say pre-90’s, some say pre-2000, but there seems to be an uncomfortable middle ground of about ’97 that gets agreed upon depending who you ask.
For a lot of people its more about a shift in focus of the designs rather than a specific year cut off.
introduction of V’s (and lack of canti’s), steel being phased out, longer suspension capable frames, 9sp components taking on ‘futuristic’ appearances and losing charm of previous shimano etc groupo’s, and generally better handling bikes are all calling cards of the end of the ‘retro’ days.