Greying here is not first about hair, though that is where the picture comes from. It names what happens to a country or a town when the average age goes up: more old people than before, and often fewer children. Families have fewer babies, people live longer, and the older group grows. Newspapers talk about a greying population, or they say a country is greying. You will also see aging or ageing for the same idea. The hair meaning still exists — His hair is greying — but this entry is the population meaning. British spelling keeps the e (greying); American writing often has graying.