Damn right, I was in Denmark a few years ago and they think nothing of running cars into their 3rd decade, import duties are so high on new cars that few people had them.
I'd change the rule a bit though, I'd have it so that you can have free tax on a second car over 20 years old, there are too many people with 10 grand restorations just to avoid paying road tax. What's bad about that is that older cars get scrapped to steal their identities for newer cars to get restored.
I'd also stopp all the classic hot rod c**p, if it's not original it's not a classic car, no cossie YB powered mark I 'tina should get free road tax