David Brenner used to joke that Florida was "where old Jewish people go to have young Cuban children" (his observation on the make-up of that society)
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Some economists believe you need a constant flux of young people - which at some level may be true..However, there is the qualitative issue of the difference between the young people being similar to the existing population...
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For older people, loud, brash young people are intimidating enough...that the young people might be culturally, racially, etc. different from them, and the society the older people grew up in, just adds an extra layer of fear, and uncertainty...
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To paraphrase, is Canada going to be "where old white people live, and have young Somali children?"