When the historic vehicle exemption was introduced it was intended to be on a rolling basis so that all cars qualified for it as they became 25 years old. Sadly this only lasted a couple of years before the plug was pulled on the rolling aspect so there the system is now extremely iniquitous and illogical, but very simple: if you can prove your car was built (yes built rather than first registered, bought or run) before January 1 1973 you qualify for free road tax. After that you don't. Of course many cars from the first half of 1973 can be proven to have been manufactured before December 31 1972 and if it is a British Car the Heritage Centre at Gaydon will probably be able to tell anyone whose vehicle is on the cusp whether they have a case.