What makes an old car a Classic car ?

I dunno. Mayne it should popular in its time. Tho I don't consider a

20 yr old a classic car yet...maybe those 240Z (Fairlady) can be considered as a classic sports car...one of the best selling sports car in history.

"popular in its time" would make all Toyota Corollas classic as they have sold a huge number of them. Only the very few DOHC Yamaha engined RWD models will truly be classics. Unlike Vintage there is no hard and fast rule as what is classic - so it really can be applied to any old car. The VSCC has laid down the definition of vintage but allow "thoroughbred" of later manufacture.

Triumph Stag is considered a "classic". They made less than 26000 in 6 years and only exported 2500 to USA between 71-73. Poor sales due to bad reputation, poor electrics, weak overdrive, water pump that would stop working if a small leak caused a small drop in water level resulting in failed head gaskets and warped heads, undersized radiator causing overheating and more frequent leaks. So it was never "popular in it's time".

They are only truly "classic" when the price bottoms out and starts going up. You don't have to give it away or scrap it just to get rid of it or it's worth as much or more than it costs to restore it.