The “under eighteens” are leaving NY because they can’t get a job?
What seems to be happening is that the under 18 population in NYC is becoming very poor or undocumented or part of a vast welfare fraud scheme.
If you are wealthy enough you take your family out of the city (Long Island, CT or NJ) to get the best schools (that are not the rabbinical schools) and more space to raise a family.
My last business trip (2016) there found lots of Orthodox kids being well schooled, a handful of good public schools and a lot of places where I couldn't make a sale without a donation to the principals’ favorite charity. Lots of kids that barely spoke English, which tells me they started learning English after the age of 6. Being white surely didn't help my sales efforts.
I do not know where the kids under 18 have gone to except to say that something is wrong in their population counts. Or the city has become too much of a hassle for even the immigrant groups.