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.
It is very expensive to raise kids in NYC because unless they can get into the exam schools, and that’s not until the upper grades, they don’t have good “schools”. That is, they have poor and unruly, too often criminal, students sufficiently populating their schools to make a decent education for decent kids improbable, at least.
So it is actually something of a status thing for the very wealthy to have lots of kids, but the middle class needs to move out to the suburbs to raise theirs. Therefore you are left with the welfare type population with plenty of kids (meal tickets) in the outer boroughs and lots of ambitious and educated young adults and otherwise educated but without school children adults actually living in the City.