We had a similar discussion a few months back.
Anyway in the true sense of the word Retro refer to something new which has been made to look like or mimic a style from a previous time/era.
i.e. New Mini styled to look like an old one, Santa Cruz Nomad with curvy top tube similar to something a lot older etc etc.
A car, bike, clothing or other object which has period styling isn't really retro if was leading edge etc in its day but rather should be classed as classic or period.
Missuse ofthe term retro though means that the current meaning can almost be anything you want as there is no hard timeframe attachd to the term (something 2 years old can be retro as can something 50 or 75 years).
In the end its not worth worrying about, in 10 years time we'll probably be refering to our bikes as "post 80s fluro" perod machines or maybe "late 20th C" to ditinguish them from the "disc brake all mountain" Retro Machines