There are more than 1.5 billion people between the ages of 10 and 25, the largest-ever youth generation. In 1999, the UN General Assembly passed a resolution making Aug 12 as International Youth Day, to highlight youth-related issues. This year's theme is 'sustainability'.

The secretary general noted the contributions made by the young people to the debate on climate change mitigation and adaptation.

The UN chief called on them to 'lead by example' in 'practicing green and healthy lifestyles, or promoting innovative uses of new technologies, such as mobile devices and on-line social networks'.