The end of the third age is the used conventionally (1340) but works from at least half a century on either side of that date are classified on a one by one basis (or by elements), and classification will vary whether it's made by linguists who have specific factors or if it's one of the classifications made by literature scholars who use more Medieval vs. Renaissance for denominations and consider factors other than strictly linguistic ones, such as aesthetics, form or themes.

Guillaume de Machaut falls with the late (troisième age) medieval poets from the literary perspective. I'm not sure if linguists call the language he used Old or Middle French or point out different elements from both. I can tell you his Le Remède de Fortune (ca 1340) still provides lexical examples and sources in my old French dictionary and a later work like his Le Voir Dit appears in my Middle French one.