Summary: This article discusses the age qualifications for serving a full-time Mormon mission for young men, young women, senior couples, and Church service missionaries.
Single men between the ages of 18 and 25 are eligible to serve a full-time mission for the LDS Church. These young men are generally called to serve for 24 months. Once single men reach the age of 26, the Church will no longer consider their application to be called as a missionary.
Young men and women should also be worthy and prepared to serve a mission. For more information, see my article on what it means to be worthy to serve a mission and my other article on requirements to serve a mission.
Single women are eligible to serve a mission if they are 19 years old or older. The eligibility age difference between the young men and the women is to emphasize that full-time missionary work is a priesthood duty of the men, while women are not under that same obligation. Though not a duty in the same sense as it is for men, women make a valuable and unique contribution in the mission field, and the Lord needs and welcomes their service.
Single women between the ages 21 through 39 are usually called to serve for 18 months. Single women over the age of 40 are usually called on non-proselyting missions (temple missions, welfare missions, office support, family history, etc.) and serve for 12 or 18 months. For more information, see the sister missionary page which has links to many sister missionary related topics including advice for young women considering a mission, and an article from former sister missionaries explaining what made them decide to serve.
There is no specific age requirement for older couples to serve a mission together. In order for married couples to be eligible to serve a mission, they must no longer be be working full-time and, if they will be serving away from home, they must not have any dependent children living in their home. Senior couples can be called to serve for 6, 12, 18, or 24 months depending on their capabilities and the mission they are called to fulfill.
Church-service missions are a good alternative when worthy men and women are not able to serve full-time missions because of health, financial, family, or other challenges. There is no maximum age, but men should be at least 18 years old, and women should be at least 19 to be a Church-service missionary. Church-service missionaries can be called for a term of anywhere from 6 to 24 months. They usually work between 8 and 32 hours a week and live at home while they are serving.