What is the ideal SE site to ask about population growth and demographics in general?
What is the ideal SE site for xxx ...
Use a search engine with "stackexchange xxx" to find a site, then search on the tags to accumulate a list of statistics, compare the list with your subject and check each site's help file to determine where best to ask your question. You need to search for duplicates, while doing that you can view a few questions to determine which site seems to give better (or faster?) answers to the category of question you are asking.
Economics has 8 questions tagged demography, with nothing for population or growth.
Decide if your question about "population growth and demographics, in general" should have a bias for biology, economics, GIS or history - if none of those fit you can propose a new site or try to reopen a failed beta at Area 51 (as Glorfindel suggested).
I remember that there was an Area 51 proposal "Geography" where this would have fit, but it has been closed. I don't think there's currently a site in the network which covers these topics. You could try to re-propose Geography, but the bar for new sites has been set considerably higher.