There are 2 main points here.
1) Age, a car can be a classic pretty well at any age, it depends on the car the prod run the desirability the "coolness" instinct, age really isn't the main factor, for example the new Fiat 500 (as opposed to the old Fiat 500, that was actually 500cc, 498 for the pedants) is pretty well a classic now. My own 300zx is considered a classic as is the BMW 5 series alpina B10, I could go on however the point is that age is not the deciding factor.
2) Free"classic car" road tax. Now in this context the word "classic" is merely a government term whereby, originally at least, it meant any car that was 25 years old. This system was introduced under the Major government as a rolling 25 years and as such if it had stayed thus then cars from 1987 would be qualifying for this year. However, a certain Labour government froze the date in 1997 and thus 1972 remains the cutoff date for free tax.