for the Church of Jesus Christ of Latter-Day Saints
s) is voluntary, and requires that paper work be filled out and personal interviews
made before applicants are accepted to represent the Church
Single male missionaries must be at least nineteen, and single female missionaries must be at least twenty-one. The men serve for 24 months, although this period can be extended for a few months and vice versa; The women serve for 18 months. Before missionaries embark to their assigned mission they spend at least three weeks in the MTC(Missionary Training Center) generally the one located in Provo, Utah on Brigham Young University campus. While at the MTC and in the field, each missionary has a roommate, or companion, whom they spend most of their time with. These are periodically switched every 2-6 months.
If the missionary has been called to a foreign language-speaking mission he or she generally stays in the MTC for a longer period of time to learn the language in an intensive study program. Besides studying language, culture and manners, the missionaries learn more about their religion as they study the scripture almost completely unique to their religion, The Book of Mormon.
The missionaries learn six sets of "Discussions" which teach the main principles of the gospel including their faith in God, the Eternal Father, his Son Jesus Christ, and the Holy Ghost; faith in the latter-day prophet Joseph Smith, baptism and eternal family relationships. These discussions are memorized to maintain the pure doctrine and are simple to understand. The discussions are based on personal prayer. The person investigating the religion is challenged to pray and personally reflect on what the missionaries present.
If the investigator decides to listen and participate in all six discussions, then he or she will eventually be challenged to baptism.
The missionaries, however, do not only teach discussions. They are required to do community service, and some missionaries work solely at Mormon temples, and do not ever go door-to-door. (I have just learned there are missions solely for service--without proselyting or temple work.) This is common for adult LDS couples and single adult women who serve missions. I don't know what age requirement there is there, but they generally serve for a period of 18 months, although it is very easy to reapply for a longer term.
Missionaries are required to dress in the business attire of the country they serve in, wear name tags, and be addressed by their last name with "Elder" or "Sister" as a prefix. Single missionaries pay about four hundred dollars a month to serve. They are primarily teachers and tools of service for the communities they serve in. In many countries they teach English, such as Japan. A mission is not required but is heavily encouraged as it forces members of The Church of Jesus Christ to study their religion, focus on serving others, and be independent.