Aging populations in developed markets has been a concern for some time, but population growth is also slowing in some emerging markets and working-age population shares are falling everywhere but Africa. “We see Africa’s voice in global deliberations—and the role of African financial markets—as likely to be increasingly important,” write Citi analysts Nathan Sheets and Robert Sockin. Its share of the global population increases from 15 percent to 20 percent, and the relative size of its workforce rises while dropping everywhere else. It’s clear that the percentage of a country that is actually at work impacts GDP, but Sheets and…