The question arises from the attached chart I prepared today.
On the importance of the distinction between gerontogrowth and population ageing, I invite those interested in the subject to see the article "Gerontogrowth and Population Aging in Africa and the Global Age Watch Index," published in 2016 in The Journal of the Economics of Aging.
http://www.sciencedirect.com/science/article/pii/S2212828X15300268