The full announcement can be read here. The long and short: the age requirement for sister missionaries has been lowered from 21 to 19, and the age 19 requirement for boys is now more flexible and can be lowered to 18 (for high school graduates) under appropriate circumstances.
There are lot of practical implications that flow from this change, but I want to focus on one thing in particular. It seems strange or maybe a bit silly that at the same time that they equalized things by lowering the age requirement for sisters, they re-unequalized things by lowering it for elders. There’s more at play here, though. First, they technically didn’t lower the official standard age for boys, they just made it easier for boys to go earlier when necessary (the examples of mandatory military services in various parts of the world were specifically invoked in explanation for the change). But it would seem like overkill to lower the sister age to 19 and then emphasize the same potential exemptions. On the one hand, to the extent that military service really is a driving concern behind the 19-to-18 change, it makes sense that Church leaders think of it only as applying to young men. On the other hand, flexibility and possible exceptions are more necessary for boys precisely because boys are still expected to serve, and girls aren’t.
That, it seems, is the key. The discrepancy here isn’t really the age requirements. Rather, the difference in age requirements is just a practical consequence of a deeper discrepancy that is still just as present after the announcement as it was before the announcement: we require mission service from young men, whereas it is optional for young women. Don’t get me wrong—I think the 21-to-19 change is a very big deal, mostly because it will, in fact, very likely encourage a much larger number of young women to serve. I really do expect the sister missionary numbers to sharply spike in coming years. But the underlying difference still remains. In this vital, critical, world-preparing work of preaching the gospel and gathering Israel and flooding the earth with the Book of Mormon, we absolutely need as many worthy young men to serve as we can.
Young women? Well, if you want…