He was speaking at the premiere of his documentary about environmental issues, Before The Flood, at the London Film Festival.
The film sees the Oscar-winning actor travel the world to see the impact of global warming first hand.
He says there are many things young people can do, but the most important is to use their vote.
"I think this is the issue of [young people's] generation and there has been a huge movement out there from the youth," he told Newsbeat.
