The use of hyphen in "twentieth-first-century reader" and "twentieth-first century reader" [closed]

Ordinal numbers — those that order things first, second, third, erc. — are formed after even tens by making only the second number an ordinal:

forty-fourth president, twenty-first birthday, seventy-fifth anniversary

As for the hyphenation, the Chicago Manual of Style recommends hyphenation with numbered centuries when used attributively, i.e., before the noun modified, but not predicatively, i.e. after a linking verb:

century:

• the twenty-first century

• the twenty-first century

• fourteenth-century monastery
• twenty-first-century history
• a mid-eighteenth-century poet
• late nineteenth-century politicians
• her style was nineteenth century

Many writers follow this recommendation:

No single modern English word will convey all of these meanings to a twenty-first-century reader, and substituting a phrase may destroy the intended ambiguity.

… but it will provide a list of novels and non-fictional works that may have been missed, even by the most conscientious twenty-first-century reader.

And yet, while the distinction appears clear enough in principle, and is no doubt familiar to the twenty-first-century reader, Goethe's essay reveals a striking complexity in terms of its practical application.

Is the twenty-first-century reader facing a crisis of cultural confidence like that of the author in the twentieth?

Other writers, however, do not use a hyphen before century. The reasoning is that a reader is not likely to understand, for instance, this book as one of several publications inexplicably all numbered twenty-first of a series named Century Readers:

It would be quite difficult to misconstrue the following sentences. The second hyphen is optional, i.e., a convention, not a rule.

… to entertain rather than challenge the faith of the reader, as well as to appeal to the cynical twenty-first century reader versus the transcendental nineteenth century reader.

Not only a twenty-first century reader, but also a nineteenth century reader could have pointed out just how undesirable this situation is.

As a twenty-first century reader of the 1839 Constitution we can easily see how it is time-bound—a product of its culture.

The Seren series, entitled New Stories from the Mabinogion, takes these ancient tales and retells them for the twenty-first century reader.

So unless your writing must conform to a particular stylesheet — or the preference of an instructor — you are free to choose whichever form you wish.