Priesthood in Latter-day Saint usage is not a class of person but a delegated authority, and it comes in two orders with a clear division of labour. The Aaronic or lesser priesthood, named for Aaron, holds what the movement calls the keys of the ministering of angels and of the preparatory gospel. Its three offices are held by young men from the age of twelve: deacons pass the sacrament to the congregation, teachers prepare it and assist in ministering, priests may bless the sacrament and perform baptisms. A twelve-year-old carrying a tray of bread along a pew is exercising priesthood in the movement's own understanding of the word, and that is not a devotional flourish but the literal position.
The Melchizedek or higher priesthood, named for the king of Salem who blesses Abraham in Genesis, holds the keys of the spiritual blessings of the church. Its offices are elder, high priest, patriarch, seventy and apostle. It governs confirmation and the gift of the Holy Ghost, blessings of healing by anointing, the temple ordinances, and every presiding office above the ward level. A man ordained an elder at eighteen may then serve a mission, and later be a counsellor in a bishopric, a bishop, or nothing more than an ordinary member for the rest of his life, and the ordination is unaffected either way.
Both orders are conferred by the laying on of hands by someone already holding them, and a Latter-day Saint can usually recite the chain by which his own ordination is traced back to 1829. Authority is understood as a line, not a status.