This method is only a rule of thumb, and can be off by a couple of centuries; it should always be backed up by documentary evidence, if possible, and take into account other factors. Caveats include the fact that planted hedgerows, hedgerows with Elm, and hedgerows in the north of England, tend not to follow the rule as closely. The formula also does not work on hedges more than a thousand years old.
According to BBC History, Dr. Hooper first developed this scheme in the 1950s. It is important not least for its potential use in determining what an important hedgerow is, given their protection in The Hedgerows Regulations (1997; No. 1160) of the Department of the Environment, based on age and other factors.
One derivative or corruption of the formula, given by Chris Brown though not necessarily of his concoction, is to multiply the number of species by 99 years and then deduct sixteen.