This is a thoroughly weird announcement.
i think the clue for its reason is in the last part, if someone is under those ages but already appointed, they can continue in their role.
My take:
I think with all the recent emphasis on involving young brothers, even unbaptized ones, in congregation activity, and the recent WT article on how a CO could be appointed “even in his mid 20s”…..I think some / many / a lot of congregations went completely overboard and started appointing 14 year old MS and 17 year old elders.
Someone “at the top” with a few remaining firing synapses realized how ridiculous it was to have teenagers giving marital counsel or child-rearing counsel or whatnot to people old enough to be their parents, or even grandparents. So they finally realized, after literally 5,6, 7 decades of repeatedly stating “the scriptures do not set a minimum age”, they decided they had to “go beyond the things written” and….set a minimum age.
That’s the only thing I can think of to prompt this abrupt and clumsily worded pronouncement.
It also occurred to me there might be some legal considerations as the reason for it, but if that were so it seems they wouldn’t allow those child MS and child elders to continue if already appointed.