Technically, The Aeon dawned with Crowley back in the early 1900's. However, there is no clear succession of these ages. They are generally marked by significant changes in the world.
The previous era was the Aeon of Osiris (Jesus/Dying God). I'm not sure if you're familiar with the Denver Airport Mural? The
image with the little girl who is in the casket holding a cross? It's symbolic of that age ending. So, although it started in the early 1900's it hasn't come to fruition until now.