There are new ones beginning all of the time:

I just found two more today by surfing the net for "global youth"

The UN Office for Drug Control and Crime Prevention
http://www.undcp.org/global_youth_network.html

The World Conservation Union's Global Youth Reporters Programme
http://www.iucn.org/info_and_news/gyrp/

World Youth Organization on Climate Change
http://www.wyocc.org/

I've listed a bunch at the bottom of this page:
http://www.youthmovements.org/analysis.htm

... more to come ...