If we want to get nerdy and specific, I've always considered
73-79 - the foundation era, hip hop figuring itself out.
79-87 - old school era, hip hop has grown to a point of having more than regional stars, it is becoming good business. It has a distinct sound but not a whole lot of variety.
87-92 - the lyricism era, rappers like rakim and big daddy Kane make it a lot harder to be considered a good rapper by raising the bar and rapping like nobody else before them.
92-97 - golden era where rap was becoming big money and the amount of variety was accompanied by a deluge of hungry young rappers eager to make it.
But again, just my interpretation.