I was wrong about the start date, BTW. I'd thought that fertility started dropping after 9/11 (which also roughly coincides with the children of the 70s having kids instead of baby boomers, a smaller cohort), but it really didn't peak until 2007-2008. Then there's a double-whammy: there are fewer women of childbearing age (coinciding with Gen-Xers born in the 70s and early Millenials), and the fertility rate among women who are of childbearing age has fallen off a cliff. Births are down about 15% since 2007; when you think about what that means for a typical classroom (where you might've had 24 kids, now you might have 20), that's pretty huge.
Looking at the first chart tells me that the size of the upcoming cohort will be similar to Gen X -- that is it will be smaller than the previous but not unsustainably small.
Although that chart doesn't capture the ratio of women of fertile age in each year, so there may be an unforeseen cliff.
