[First Presidency Letter]
We hereby approve the Twelve's recommendation in letter of September 29, that a definite time be fixed for the ordaining of Priests to the office of Elder in the Melchizedek Priesthood, and that this age of 19, thus affording Elders the privilege of two years of training and experience before they become eligible for ordination to the office of Seventy.
We also authorize the adoption of the following schedule, as submitted in your letter above referred to, for advancement in the Aaronic Priesthood:
Three years experience and training for the Deacons'12, 13, 14;
Two years experience and training for the Teachers'15 and 16;
Two years experience and training for the Priests'17 and 18.
[First Presidency, Letter to Rudger Clawson, as quoted in Minutes of the Apostles of The Church of Jesus Christ of Latter-day Saints, 1910-1951, Privately Published, Salt Lake City, Utah 2010]