According to the Wikia page, Halliday was born in 1972 and died in 2039, making him just 67 years old. This is somewhat preserved in the movie, too; we’re told by the end of the movie that he was 7 during the early 80’s and that he died in 2040, so he’s at most in his late 60’s-early 70’s when he died.
With medical advances prolonging the average life span, even back in 2011 when the book was written that was awfully young to have died of old age. And he didn’t just die of a heart attack, either; the movie specifies in several places that he was anticipating his death. So, at least in the movie version, was medical care just thrown out the window with the poverty of 2045, or did he die of some obscure disease that I missed the reference to?