I understand why they use that phrase. Partly it is to find a way to incorporate every person under 20 in one phrase. I can remember a CO coming through and asking everyone under 20 to stand up. They he said "these are the young ones in your midst." The WTS bases this on the account in the OT where every Israelite 20 and over was held accountable for the decision not to go into the promised land the first time.
(Numbers 14:29) . . .In this wilderness YOUR carcasses will fall, yes, all YOUR registered ones of all YOUR number from twenty years old upward, YOU who have murmured against me.
It is a phrase they take from their translation of the Bible as well.
Another word they use even more is "youth(s)."
It is an awkward phrase not used in in the NT or current speech. It is their attempt to find a word that encompasses both male and female children under 20.
While "youths" are considered not accountable in the Bible account above, another poster illustrated how "young ones" are considered capable of deciding to get baptized but not to make their own decision whether to attend meetings or not.
Blondie (still trying to edit the "pure language" out of her speech)