Where does the Morris Marina Owners Club fit in?The industry defines it as such, particularly insurers. The sales side also use the distinction as an excuse to charge ,ore.
But enthusiasts see it differently. Simply being old alone does not make a classic. There needs to be significance of some sort aside from simple age, benit historic, social, provenance, etc.
A sheet car today will simply be an old sheet car in 50 years time. However, a sheet car owned by The Queen, or used in a famous bank robbery, or that appeared in a much loved TV series, might have an argumentnfor being a classic.
The huge majority of cars (or bikes) to have come off a production line are pretty mundane and just built to do a job. That includes many of the old cars that are considered desirable today such as a MK I Escort. The sports models may have been thinly disguised rally cars but made up only a tiny percentage of sales.
If it's loved and cherished and old enough to be rare, then I would consider it a classic.