In Mosiah 2:40, King Benjamin says: "O, all ye old men, and also ye young men, and you little children." In Israelite culture, for some legal purposes, the world was grouped into old men, young men and children. With a life expectancy of roughly about forty, these three categories roughly break down into 10, 20, and 30 years of age. King Benjamin wants the elderly, the young adults, and the youth to consider the blessed and happy state of all those who keep the commandments of God.
Book of Mormon Central, "How Can the Book of Mormon Strengthen Marriages and Families? (Jacob 3:7)," KnoWhy 302 (April 19, 2017).