[thought one on this subject]
(I do not think my comments here are examples of chicken patriarchy for reasons I'll allude to, but Mormon feminists are welcome to call fowl in the comments.)
I saw a tweet during Saturday's afternoon session of, I deduce, a mother of a male BYU freshman. He (the son) and his friends were predicting that girls will soon outnumber boys in the MTC (and, I suppose, by extrapolation, the mission field). I'm doubtful, but also recognize the reasonableness of the argument as I will explain below.
I also saw a certain flunked Saint complain about the inherent inequality of (continuing) disparate ages for the different sexes. But I would like to propose, in the spirit of my previous paragraph, that the inequality we are seeing here may be a form of affirmative action. For men.
I don't know how true this may be internationally, but here in the States, our young men are in trouble. I'm not making one of those chickeny women-are-inherently-more-righteous-than-men arguments. I'm simply observing that, in general, American girls are preparing better for adulthood than American boys.
An example: Traditionally, girls have been told they're inferior to boys in, say, math and science. This problem is still real for women in the field. But do you know a girl who believes this? I just spoke with a graduating senior girl planning a career in science who had never even heard of this stereotype when I enthused about her choice. Where would she have? Most of the kids in higher-level high-school courses are female!
And she's not alone in her ignorance of an alleged girl inferiority. Nationwide, girls are outexcelling boys in pretty much everything academic. Some universities are even quietly instituting policies to let in underqualified boys just to keep the numbers closer to even. More girls are graduating high school, entering college, graduating college. The generation now in kindergarten will enter a workforce that women dominate. I don't see any other possibility.
Now let's return to the age gap in missions. I suspect that a generation of boys---used to girls being better than them---are becoming more and more willing not to take responsibility. When boys and girls are provided identical opportunities, it appears that, on average, boys step aside. I'm not saying this makes any sense, but it seems to be true. (Speaking of youth collectively and not individually.)
At this point of my on-paper drafting process, Elder Christofferson is discussing this very point: Men are falling behind. (At least in America---again, I don't know how true this is internationally.) And after I finish my draft, President Monson also speaks to this concern. (Follow the Christofferson link to watch Priesthood Session.)
No matter your feelings on men/women/priesthood, we as Latter-day Saints cannot be happy about any segment of our populace atrophying.
We can certainly disagree on whether differences in mission (starting age, length, expectation to serve) will help future Mormon men equal future Mormon women, but I think we can agree that future Mormon men had better equal future Mormon women, one way or another.
[Please complain about my arguments in the comments section to help me clarify or abandon them.]