There are two ways of looking at the border of generations. Rabbeinu Hakadosh recognized the end of an era and summed up the Torah of the Tanaim. Rabina and Rav Ashi did the same to the Amoraim. The Ge’onim ended, simply enough, with the last Gaon. The Tur did to the Rishonim what Rabbeinu Hakadosh did to the Tanaim, and Rav Ashi to the Amoraim.
We can see that the Beis Yosef doesn’t argue on the Rosh but he does disagree with the Tur at times. We live in a generation where the words of previous generations are also summed up and repackaged. We also don’t specifically argue on previous Gedolim without good company. Perhaps the dawn of the the new period begins with the Mishna Brura.
There is also the historical division — which goes hand in hand with the first approach. The Mishna ended when it got hard to continue learning in the same way that they had been doing until then. That is when we see Bavel coming into the picture. It is an obvious new era. The Amoraim ended when times changed, too. The Rishonim began with the new Yeshivos in Europe, and ended with their closing.
The Rosh escaped from Ashkenaz to Spain. After that, it is blury. The Mordcha is a Rishon, and it seems like the Maharil and his generation are also considered Roshonim. There is a continuation into the Achronim through Reb Yakov Pollak, Reb Sholom Shachna and the Maharshal. Somehow, you get the feeling that the new period begins with the Maharshal, perhaps because there is little record of anything before him.
About 150 years before the Spanish expulsion is when the situation with Anusim began in Spain. This coincides with the Tur’s generation. It came to a complete end with the expulsion. The Abrabanel, Reb Yosef Yaavetz, Ri Elbo, among other s are referred to sometimes as Kadmonim, since they don’t quite fit into either category. Perhaps, those living in this period in Europe get the same status.
In this respect, there is little doubt that the holocaust ended a period in our history. In fact, both world wars messed up the Yeshivos and the Torah situation. We started anew, and began a new era.