The U.S. Army knife was found by Ana Lucia on Jennifer, one of the Others who attacked the Tailies' camp. She believed it to be around 20 years old. ("The Other 48 Days")
Later, Ana Lucia used the hunting knife to carve a stake. ("What Kate Did")
Ana Lucia then gave the knife to "Henry" for him to cut himself loose. ("Two for the Road")
It is presently unknown who inherited the knife after Ana Lucia's death.
It is possible that the knife belonged to one of the 18 United States Army soldiers who brought Jughead to the Island in 1954, making it at least 50 years old. ("Jughead")