Diaz wrote the book as an old man in his 80s.
Well... He *finished* the book as an old man. He started the book *at minimum* 15-20 years before it was finished.
One of the prefaces for the book mentions a “daybook”, so Diaz may have kept a diary. He almost certainly was in close contact with former comrades for the 30 years before the first mention of him working on the manuscript in 1552.
It is clear that he considered the account to be valuable, so he likely took notes from conversations he had with people like Pedro Alverado, who was one of Cortez’ Lieutenants, who he served under several times. Alverado became the Governor of Guatemala in 1524.
Well... He *finished* the book as an old man. He started the book *at minimum* 15-20 years before it was finished.