Why do young men start home teaching when they’re 14 but young women start visiting teaching when they’re 18?

In a revelation given at the time the Church was organized in 1830, the Lord declared, “The teacher’s duty is to watch over the church always, and be with and strengthen them” (D&C 20:53). Besides teachers in the Aaronic Priesthood, this duty is shared by priests and by those who hold the Melchizedek Priesthood (see D&C 20:45–52). Home teaching is one way they fulfill this duty by accepting specific assignments from priesthood leaders. Visiting teaching, although similar, has a slightly different purpose, providing an opportunity for Relief Society sisters to strengthen and teach one another through assignments from the Relief Society presidency (as approved by the bishop or branch president). Young women (especially class presidencies) should, of course, seek to strengthen and support one another in a variety of ways, but they are not assigned to make monthly visits to specific people.