A time-based definition is long overdue. After all Vintage and Veteran cars are defined on this basis.
The government exemptions are based on forty years, perhaps that could be used as a starting point.
There are several different definitions world-wide which seem to vary from 20 years upwards.
Defining a "Classic" on the basis of rarity or desirability is a non-starter in my opinion.
Insurers also vary in their application. I've had a ten year old car insured as a "modern classic" - whatever that is.
