"In addition, eight deaths were listed as caused by "Decay" and eleven deaths were listed as caused by a "Wearing", "Long Affliction", "Pain and Sickness", "Afflicted 10 years", "Long Disease", "Disease and Always", "Feverish Decline", "Awaisting", "Weakened", "Cronical" and a 6 year old who is listed as dying from "3-year-old illness". I feel that these deaths could be described as "Decline"."
I would therefore assume it was an illness that he had had for some time and it gradually wore him down !
His twin brother also died at the age of 21 of Consumption. Sad.