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.