Each year the Sunday School presidency and a member of the bishopric determine which of the approved classes to hold that year. . . . Class members are grouped according to their ages as of 1 January, when the curriculum year begins. Youth normally advance to the next class on the first Sunday in January.
When children turn 12 years of age, they begin attending their Aaronic Priesthood quorum or Young Women class during Primary sharing time. However, during Sunday School, these 12-year-olds continue to attend the same Primary class until the following January. This class becomes their Sunday School class.- Instructions for Priesthood and Auxiliary Leaders on Sunday School