The answer to this question can be found in Chapter 15. Diamond does not explicitly say when the Aborigines began their history, but we can see in this chapter that he thinks the Aborigines began as a separate people between about 13,000 and about 10,000 years ago.
Diamond speaks of this time frame as the time in which Aborigines became very different from the people of Papua New Guinea. He says (p. 297 in the paperback edition) that there was "less cultural change" in Australia in the past 13,000 years than anywhere else. That implies the Aborigines became separate around then. Later (p. 302), Diamond mentions that the Arafura Sea completely separated Australia and New Guinea "around 10,000 years ago." Thus, this is the time frame in which Australian aborigines became separate from the people of New Guinea.