I don't think they talk about it in the (classic) cartoons of him, but in the 1995 movie "Casper" he talks about it. He didn't say what year but he told Kat Harvey (the girl he had a crush on) that he died when he was 12 years old after staying out in the cold too long when he was playing with his sled he got for christmas. He said: "he was having so much fun he didn't want to come inside." He got sick and died.
