Not how that works, sorry. Era is a relative term, not a unit of time. Without being qualified relative to a person or series of events it has no set meaning.
A period of time when pretty much when the best fighters careers are coming to an end or are at the end that marks the start of a new era IMO. We are in the start of a new era now.
Depends on whether you want to use the guy on top as the measuring stick (in the case of a dominant, long-reigning champ), or when the majority of contenders in the top 10 rolls over to the next group. Using the former, for example, most of the 2000's has been the Wlad Klitschko era. Using the latter, he took out nearly 3 generations of contenders (most of the ranked guys now weren't even pros when he took over). It seems more popular/convenient to define it by the dominant champ (if there is one).