We thank Wordsworth and Coleridge for publishing in 1798, which is close enough to a century boundary for all practical purposes, and lets us divide our anthology chapters and survey courses at a good round number. Doubtless it would do something. But dates, however important, aren't the whole story. But not all writers were so considerate of later historians in choosing when to live and die. Renaissance scholars claim Milton as their own, and eighteenth-century scholars have taken Denham in return, even though Paradise Lost appeared in 1667 and Cooper's Hill in 1642.
Literary periods can be surprisingly elastic.
This matters for several reasons. But more important, periods are all about identity. To decide what is eighteenth-century is to decide what the eighteenth century is. But rarely is the dynamics of cultural periodization given much thought. But few have examined the received periodization of seventeenth-century literature sympathetically, trying to understand the implied characters attributed to the two ages by the very act of associating writers with them.