According to the writers, Aang died due to drained life energy and health complications caused by the 100 years he spent trapped in the iceberg. He was biologically the youngest team Avatar member to pass away.
Last photo taken of Frederick Fleet, the Titanic lookout who spotted the fatal iceberg. Fleet would massively struggle with survivor's guilt and bouts of depression right up until his death. He ended his own life in 1965, aged 77.