Starting with the elderly, as they naturally pass away by virtue of being human, and being old. No-one is suggesting that we actively kill people. The article

Starting with the elderly, as they naturally pass away by virtue of being human, and being old. No-one is suggesting that we actively kill people. The article is talking about attrition (rates of reproduction are declining - fewer people are being born and the people who are here naturally age & die) From TFA: "The next stage of the demographic transition, after a drop in mortality, is a subsequent drop in fertility (although the relationship isn’t straightforward). Family sizes fall from 5 to 6 down towards replacement level. But then they drop further — and never come back. Indeed across most of the world, outside of sub-Saharan Africa, family sizes are still shrinking, and populations ageing."