When did the church first start sending out 19-year-old missionaries?
The First Presidency issued a statement on July 21, 1960 allowing young men to serve missions at age 19, even though they had not met educational and military qualifications previously required. In November 1961 they established a Language Training Institute at Brigham Young University. In 1963, it became the Language Training Mission. A two month program was instituted for missionaries who would be learning a new language. Prior to this time, missionaries serving in foreign countries where they would be learning a new language served for 2 1/2 or 3 years. In September 1978 it was announced that missionaries would receive four weeks of training, while those learning other languages would continue to receive 8 weeks of training at the new Missionary Training Center in Provo, Utah. This program began in October 1978. The new MTC replaced the Mission Home in Salt Lake City.
Because of the war in Vietnam, in September 1965 a quota of only two missionaries per ward was established within the United States to comply with Selective Service requests. During the Korean War, older men were called to serve missions, even those who were married.
In April 1982 the term of service for single young men serving a mission was reduced to 18 months. On November 26, it was announced that the the term of full-time missionary service for young men would be for 24 months again.
These are some interesting dates concerning missionaries and their terms of service.