The other night I was reading a page from the Encyclopedia of Latter-Day Saint History on the Aaronic Priesthood. I don't know why, but knowing the history of how things came to be just helps my understanding usually, and I wanted to share. It's like when the ward leadership makes a decision and you think, what a weird decision, but then you learn later why what was done was done, and it all makes sense. Then, the next time your church leaders make a "weird" decision, you just think, aw, ok, there's probably some story behind that.
Anyway, the encyclopedia first summarizes the offices of the Aaronic priesthood and the duties of each from D&C 20, 84, & 107:
Deacon: assist Teachers
Teacher: watch over church members, prevent iniquity, lying, backbiting, evil speaking, see that members attend meetings and do their duties, conduct meetings, expound, exhort, teach, invite people to come to Christ.
Priest: preach, teach, expound, exhort, baptize, administer the sacrament, visit, teach member to pray and fulfill family duties, conduct meetings, ordain priests, teachers, and deacons.
Bishop: President of the Aaronic Priesthood
It's important to note that, "Through most of the nineteenth century, male adults filled the Aaronic Priesthood offices, assisted by a few youths." I remember learning about the priesthood in seminary and thinking that Deacons, Teachers, and Priests did the things mentioned above; however, as an adult, those responsibilities just seem a little, well, too much to handle for such young kids.
The article then points out that there were actually four distinct periods of Aaronic Priesthood history, which really makes a lot of sense getting us from priesthood duties in the 1800s to priesthood duties now.
1. 1829-1845, pre-endowment: "Aaronic Priesthood bearers were adults (except for a few outstanding youths). Their primary duty was to visit members in their homes...."
2. 1846-1877, post-endowment leads to "acting" Aaronic Priesthood bearers: Since men who went on missions or got married received their endowment after the introduction of it in 1845, and a prerequisite to the endowment for men was the Melchizedek Priesthood, few adult men were left to fulfill Aaronic Priesthood duties, so they were called to serve as "acting priests, teachers, and deacons. Some boys received priesthood ordinations...."
3. 1877-1908, ordinations broaden to worthy young men from ages 11-18: "most became deacons and stayed such until becoming elders. Few boys blessed or passed the sacrament or did what is now called home teaching."
4. 1908-Present, Aaronic Priesthood restructured "to be a priesthood for boys. They approved that worthy boys be ordained at set ages and advance through each office: deacons at age 12, teachers at 15, priests at 18, and elders at 21. Church headquarters produced lesson manuals and assigned duties geared to these age levels. For ward teaching, ordained teachers and priests served as junior companion-apprentices to Melchizedek Priesthood holders. . ." (emphasis added to show that the boys were given age-appropriate duties; i.e., the duties have changed a bit from the early days of the Church).
"In 1928 the ages of 12, 15, and 18 were changed to 12, 15, and 17 . . . with the elders' age set at 20. That age was reduced to 18 in October 1934, but by December it was raised to 19. In 1954 the teachers' age became 14, and the priests' age was changed to 16, . . . and elders were ordained at age 20 (now 18)."
It seems to me that the Church wanted to maintain someone in the Aaronic priesthood, and boys would fit the bill, plus, it was the perfect training ground for their future Melchizedek Priesthood service.
I think changes happen so infrequently these days, that it seems nearly utterly shocking when things do change, such as the relatively recent changes to missionary age. Also, duties themselves can change, as they have over the years as mentioned above. One interesting example of duties changing actually involved girls in 1945! The Church News from April 21, 1945 reports that in the SLC 24th ward that there was a shortage of men/boys to collect fast offerings, so the Beehives were assigned to do it for two years. This tells me there's nothing inherently tied to boys collecting fast offerings, it's merely an age-appropriate assignment that helps prepare them for future responsibilities.