In a small town in Italy or Japan, the primary school closes for lack of pupils; in a district of Dakar, it turns children away for lack of desks. The world is ageing on one side and growing younger on the other, and the two movements will together decide the employment, the pensions and the peace of the century. Macky Sall places this question in the first rank of his candidacy for the office of UN Secretary-General.

A world that is ageing. According to the United Nations, the number of people aged 65 and over will double by 2050 to exceed 1.6 billion. Across much of Europe and East Asia, births no longer replace the generations, and the working population shrinks year after year.

A continent growing younger. The median age in Africa is close to twenty, against more than forty in Europe. Each year, millions of young people there enter the labour market. That demographic dividend makes a nation's wealth when school and jobs follow; it makes its instability when they are missing.

Two needs that answer each other. Ageing societies lack workers and contributors; young societies lack jobs and capital. Organised cooperation, through training, investment and managed mobility, turns that difference into a shared strength. Left to chance, it feeds tensions on both sides.

What the Secretariat can carry. The Secretary-General can give demography the place it does not yet hold in negotiations on employment and mobility, and bring ageing countries and younger ones together around shared training programmes. Population policies will remain the business of each state. The instrument already exists, the World Population Prospects published by the United Nations, which give every government the state of its own age pyramid.

Experience in multilateral negotiation. Twelve years as head of state and the chairmanship of the African Union in 2022 underpin these commitments. That experience explains why the vision statement filed with the United Nations frames its proposals as obligations: peace, security and development conceived together; a renewed multilateralism; stronger governance of the Organization.

Africa at its heart, the world in its sights.

The United Nations' World Population Prospects are public. Read all public positions at www.mackysall.net