I'm relatively new to AV. I started about 5 years ago but didn't start actually keeping videos until 3 years ago. At that time, Ai Uehara, Yui Hatano, Hibiki Otsuki and Haruki Sato seemed to be the biggest names. Between then and now, Ayumi Shinoda, Aika and Kaho Kasumi became more popular.
So my questions are:
1) Are AV generations defined by a set number of years?
2) How do you break up a generation of AV?
3) How would you break it up based on stars?
4) Do you break it up based on advancements in technology?
5) Would you break it up according to themes or direction?
It's hard to really answer these Q's. some newbies retire and fade away after a year or two after their debuts and some last a decade with few hiatus/comeback. It really depends if you're talking about dividing girls according to their debut age/year or in other ways.
Original ebisu muscat and current ebisu muscat can sort of draw a fine line between two eras except the leaders of each group are a bit older. Original-Sora aoi, yuma asami, akiho, rio, mihiro, aino kishi, and many more that started in the early and late 2000's and lasted at least 7 years each. Honorable mentions like Kaera Uehara and Saori Hara(minor ex-celeb before av) sort of fit in this era as well.
I would say akiho yoshizawa would be the highlight and last of the original av idols of her era, which consist of Maria ozawa, yua aida, sora aoi, etc. This year would be her 15th year in the biz and she's still fairly moderately popular after all these years. Her era sort of began the social media/events communication and connection with fans. Even if an popular av actress retire, she would still be socially active except for ai uehara. Yuma Asami and Mihiro are kinda aiming for a singing and hosting career. Sometimes, they go live on twitcast streams.
The current era/upcoming era might be reserve for ex-idols. Shoko and yua are definitely the highlights of the current era. They receive a lot of outside opportunities and are also the top bestsellers of 2016 due to their past scandal as idols.
Technology? Does HD/Bluray, thinner mosaic/thinner pubic hair, and longer/rougher sex scenes counts? There are more mainstream nasty rxpe series, elder series, little people joining the 70 av members(most are also amateurs too)