The number of young people in the world has never been higher. In all, there are about one billion 12 to 18-year-olds. Almost nine out of 10 live in the developing world.
The proportion of young people per country is highest in Africa and lowest in Europe. In Uganda 57% of people are under 18. In Italy, Greece and Spain the figure is 17%.
Developing countries which invest in their young could see economic growth surge, says the World Bank. Those which fail could see tensions.
A quarter of young people live on less than US$1 a day.
Although adolescents are the largest section of society in many countries, the influence they have is often limited.
In some countries, parents govern their children's lives well into adulthood. Only one in 25 Bangladeshis decides who they will marry, while almost half of Tajiks do not choose their own job.
In many wealthy nations young people are living at home for longer because of financial pressures.
In most countries young people have to wait until 18 to vote. Iran's voting age is lowest, at 15.