Premise: Nothing can stand in the way of Jesus’s saving power. If a physical ordinance is needed to connect a person with this saving power, it must be available to anyone who needs it: which is everyone past the age of accountability.
Since the Church will not baptize children of same-sex married couples until those children reach the age of majority, move out of their parents’ household, and disavow same-sex marriage, one of two things must be true:
Either getting baptized into the Mormon Church at age 8 is not necessary to salvation (or even important to it), or the LDS Church is not the only institution that has the power to bestow ordinances that connect one with the saving power of Christ.
Assuming that the LDS Church is the only authorized institution to perform baptism, it seems that nothing spiritually important is gained from baptism until the age of majority. Otherwise, the LDS Church could not be the only purveyor of this saving ordinance since it must be available to everyone who needs it.
Using the same line of reasoning, since the Church also denies these children confirmation and the gift of the Holy Ghost, the light of Christ must be perfectly well suited to guiding a person through the first 18 years of life.
Therefore, 18 must now be the age of accountability, at least in the United States, because that is when the Church is willing to offer baptism to everyone.