Missionary service shapes the movement at least as much as it shapes anyone it converts. About sixty thousand missionaries are in the field at any time, and the great majority of them return to ordinary life having spent two years speaking about their religion every day to strangers, most of whom were not interested. A returned missionary is fluent in a language, accustomed to rejection, trained in a fixed daily discipline and known in a congregation as someone who has done the thing everyone is expected to do. It functions as the movement's rite of passage into adult standing, and it does so more effectively than any ordinance.
The age change of October 2012 shows how much rests on it. Lowering the minimum age from nineteen to eighteen for men and from twenty-one to nineteen for women was presented as an administrative adjustment; it nearly doubled the missionary force within a year, and the number of women serving rose several times over, because the old age of twenty-one had fallen in the middle of the years when Latter-day Saint women typically married. The consequence was a generation of women with mission experience entering congregational leadership, which is a structural change to the church that no doctrinal statement announced.
The practice has costs the church has acknowledged. Missionaries are young, far from home, working long hours under pressure to produce results, and the mental health of missionaries has been the subject of open discussion and of changes to policy on rest, on contact with home and on returning early without stigma.