Fertility rates all over the world have been falling but they are uneven.
Right now there are 7 billion on the earth and you can broadly break that number out to 1 billion in the Americas(N&S). 1 billion in Europe and Russia. 1 billion in Africa. 4 billion in Asia.
By 2050, Africa will add a second billion and Asia will add a 5th billion for a 9 billion total.
By 2100, Africa will add a 3rd and 4th billion for a total of 11 billion.
The CIA factbook publishes a list of the national fertility rates, in descending order. Also the Wikipedia page on fertility rates publishes the CIA list plus those published by the World Bank and the UN.
The figures I was questioning was exactly who was breeding in the Americas (in the US specifically); many adults aren’t even breeding anymore.