If the job market (and economy) was zero-sum then why is population growth generally considered good for the economy? Wouldn't young people, as they age into t

If the job market (and economy) was zero-sum then why is population growth generally considered good for the economy? Wouldn't young people, as they age into the job market, push out all older workers? This happens in anecdotal cases but overall the vast majority of older Americans have jobs despite being outnumbered by younger generations. We realize that new generations not only take up jobs but they also add value to the economy, growing it and making it possible for everybody to have jobs. Except in isolated cases immigrants do the same thing. If this wasn't true then America--being largely a nation of immigrants--would be one of the most poor countries in the world instead of one of the richest.