So the announcement that youth would rotate up in each January came as a surprise to a lot of people. Here are my first thoughts on the matter:
I have heard more concern expressed recently that children in Senior Primary needed to be getting more attention than they were. This pushes the eldest part of that group into the youth organization and under the eyes of the Bishopric and mutual leaders.
The new approach creates more class size stability that could help wards manage resources better. Essentially every year Bishoprics and Presidencies look at class and quorum sizes and ask “is this too small to function well?” If so, think about how to fix it; as opposed to the more ad hoc approach in the past where it seemed like a quorum or class needed reorganizing every six months or so. Presidencies are more likely to be called and rotated annually. Hopefully this inspires some February Handbook training to become a thing.
It cleans up the weirdness with the new two hour block having the 11 year olds bounce between Primary and YM/YW from week to week. That was going to be an issue since Primary is now subdividing their time so the 11 year olds would miss half the singing times and so forth. Now they are either with the other YM/YW or they are in Primary.
More generally, youth will now have more of the same kids in their class each week as they move from Sunday School to Mutual and back. It might open the door, when needed, to calling some youth leaders/advisors into Sunday School so that youth get more continuity in their interactions. As it is, a quorum or class advisor who in the past was solely responsible for Sunday teaching is not going to be very busy.
A variety of personality and age issues that were mitigated by the rolling window are now going to be worse while others get better. A child who is the only one in his age cohort will not at least get to overlap part of the year with others. Two youth who do not get along may now end up in the same class the whole year, or not at all. Similarly, two youth that interacted well together may no longer be together. These effects seem to me like they will largely be idiosyncratic, though, helping and hurting in an almost random fashion from unit to unit.
Mass ordination in January! Along with a more obvious need for training each year as to Priesthood responsibilities. On the old calender, a poor new Deacon sometimes just showed up at the pew after ordination and got some whispered instructions. Now a well functioning quorum could actually plan to do some training each year on priesthood responsibility since a year’s worth of boys will be rolling over. Similarly, December/January marks a transition period where one can set goals and discuss Duty to God or Personal Progress in a more systematic way.
Priests and Laurels will be there for a longer period. My son was a Priest for basically three years. Now it would have been even longer.
We have a standard of behavior/ability for a new deacon/beehive and there may be some adjustment since they will be younger now than before. Perhaps that standard was not particularly high…
The scriptures give no age for priesthood advancement and I honestly don’t know when we standardized on the 12/14/16 that we use (update: Now I know. See here from the FAQ for the modern history of age changes). But it is cool to see the ages change even if for no other purpose than to help us realize that many policies are mutable and subject to ongoing revelation.