Brás Cubas died at about sixty-four. He says so himself, in the first chapter: he expired at two in the afternoon on a Friday in August 1869, at the Catumbi farm, and "was about sixty-four, sturdy and prosperous". He was unmarried, possessed about three hundred contos de réis, and eleven friends accompanied him to the cemetery.
The count matches the birth date the book gives later: Brás was born on October 20, 1805. Since he died in August 1869, two months were lacking to complete sixty-four, which is why he speaks of "about" sixty-four.
According to him, the cause was pneumonia caught while he was busy with the plaster he wanted to invent, but he prefers to say that an idea killed him: the anti-hypochondriac plaster, which would have brought him fame and money and which never came to exist.