Median age of country populations

Interesting map! Why median? What are the implications? 

https://statisticstimes.com/demographics/countries-by-median-age.php

Demography is the study of populations. Scientists try to look at the big pictures of our world, its populations, and the resulting implications.  One thing that has been noted is the median age of countries around the world. What are the ramifications of a predominately young population or a predominantly older population,  Why do scientists use the median measurement to study this?

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