It's a bloody good job the age rule doesn't apply to when it was 1st sold to the public , if that was the case then most of the bikes on here that are "retro" would be classed as modern due to the high percentage of NOS.
In my view retro is getting hold of (or having owned for many years) an old bike , namely one that was sold into the market of general usage not NOS and keeping it working and enjoying riding it . Some people take life far too seriously in my opinion.
As to the analogies to the classic car scene I can build anyone of you a brand new MGB/Midget/Mini , is it a classic because its built from totally new parts , in the eyes of the DVLA yes but not in the eyes of a lot of people.
Perception is different for everyone , some people have open minds , some are blinkered but hey ho, sometimes I ride modern (that looks retro as it's steel with straight tubes) sometimes I ride retro with some modern bits on it, the bottom line is I go out for a ride and a laugh with friends, some of which I've made on here.
Matthew