Sub-Saharan Africa faces three big inter-related challenges over the next generation. It will double its population to two billion by 2045. By then more than half of Africans will be living in cities. And this group of mostly young people will be connected with each other and the world through mobile devices.

Properly harnessed and planned for, this is a tremendously positive force for change. Without economic growth and jobs, it could prove a political and social catastrophe. Old systems of patronage and of muddling through will no longer work because of these population increases....