President [N. Eldon] Tanner presented the request of Elder Theodore M. Burton [of ] the Genealogical Department for instructions in the matter of sealing living persons by proxy as in cases where the living person is mentally or physically so incapacitated that he cannot come to the Temple for the ordinances, and also in cases where the mental age is not determined. He reviewed the ruling that when a person is not physically or mentally able to go to the Temple the work may be done by proxy. Problems included are: Who decides whether the mental age is eight years or over? The Genealogical Department has been informed that a competent physician may determine the mental age and if it is not eight years of age the child is not baptized, but that such a child may be sealed to parents. I directed that each case be considered on its special circumstances.
[David O. McKay diary, emphasis in original as quoted in Anderson, Devery; The Development of LDS Temple Worship, 1846-2000: A Documentary History, http://amzn.to/TempleWorship]