Many people who hear about the Mormon priesthood confuse it with the priesthood of other religions. The priesthood of the Mormons is different in many ways and the differences are important to understanding many aspects of the religion. Mormon is a nickname for members of The Church of Jesus Christ of Latter-day Saints.
The Mormon priesthood is not a career. No one can be paid for holding the priesthood because of the commandment to avoid priestcraft. It is a position of unpaid service to God and the authority to act in His name in certain circumstances. It does not convey any sort of power in the worldly sense.
The Bible tells us of the priesthood of Aaron. There were many rules attached to this priesthood. Only men could hold it and those men had to be of a specific lineage and without blemish. These rules were made by God and were unchangeable by mortals. The requirement to be without blemish was to represent the perfection of Jesus Christ when He came, something to keep them focused on that. Today, since He has already come, that and some other aspects of the Aaronic priesthood are fulfilled and need not continue. However, the priesthood itself does continue.
Mormon priesthood is held only by men and boys. This has been true since the beginning of time. Jesus Himself chose only male apostles because an apostle must hold the priesthood. Jesus was not sexist. The priesthood, although held only by men, is about the type of service, not about gender. It is similar to understanding that God only gave women the right to give birth. This is also not sexist. It is just the specific role assigned to women. Some assignments in Mormonism, as in the ancient church, are given only to men and some only to women. Some are available for either gender. However, since the Mormons operate a lay church, this does not prevent employment for anyone, and both men and women can teach, preach, and pray in the church. Women hold many leadership positions, even at the international level. Elder Dallin H. Oakes, a Mormon apostle, points out that his wife held an international church position long before he did.
Mormon boys can receive the priesthood at the age of twelve. This does not make them the equivalent of a priest in other religions. Twelve and thirteen year old boys are deacons. They can pass the sacrament (communion) to members of the congregation, help care for church property (such as taking out the trash) and serve the bishop (lay pastor) as needed.
At the age of fourteen, they advance to the level of a teacher. This does not mean they teach the classes. It means they can serve as home teachers. Every Mormon family is assigned two home teachers who visit them monthly. The home teachers deliver a brief spiritual message and find out how they can serve the family throughout the month. If they learn from the family there is a serious problem that requires service from the church—such as hunger—they report it to their leaders. The teenagers are paired with experienced adults, usually their father, to learn how to carry out this important service. They will continue as home teachers all their lives. They also prepare the sacrament (communion) before the worship service begins and act as ushers if needed.
At the age of sixteen, boys become priests. Again, this is not equivalent to being a priest in another religion. The priests can bless (pray over) the sacrament, baptize, and perform the services of the lower priesthood levels. This is the final level of the Aaronic priesthood.
When men turn eighteen, they can receive the Melchizedek priesthood. This is the higher priesthood and new holders of it are called elders. The male missionaries you see are addressed as elders, as are the apostles. Other elders simply use the title of Brother, which is given all male Mormons. Mormon women are addressed as Sister. These titles reflect the relationship of all mortals as children of God.
A man must be an elder to give a blessing of healing as well as other priesthood blessings. He must also hold at least this title to serve a full-time mission or to enter into a Mormon temple. Women may also serve as missionaries and enter the title. They do not require the priesthood to do so.
Some men also become high priests. This title is required for certain types of church service, such as being a bishop (lay pastor) or stake president. (A stake is similar to a Catholic diocese.) However, when a man leaves those positions, which are part-time, temporary, and without pay, he retains the high priest level of priesthood, even if he then has a church position that does not require it. Having that title does not automatically mean the person has special authority at that time.