Neil Young seems to be speeding up rather than slowing down, despite his 62 years. He's still doing his first tour since the operation for a potentially fatal brain aneurysm in April 2005; then there's his endless political activism; and this, his first major film as a director since 1979.

CSNY Déjà Vu, as its title implies, is about the 2006 Freedom of Speech tour, which reunited Young with David Crosby, Stephen Stills and Graham Nash for only the third time since the supergroup's acrimonious split in the Seventies. Its direction is credited to Bernard Shakey, one of Young's favourite pseudonyms, but it's anything but shaky. In fact, as these things go, it has considerably less pretension and more impact than Martin Scorsese's overblown Rolling Stones pic Shine a Light, which had its international-premiere on the same day in Berlin this year.

As a piece of film-making, it's as pared down and committed as Young's music at its best. The 2006 tour, Young told me on a recent visit to Europe, was a "focused mission to get people talking again". That was the only way he would agree to reforming CSNY. "I said, 'Let's do only songs about the war and politics and the human condition.' We don't talk, we just play. No bullshit between songs, no cute remarks, no self-aggrandising political statements. Only the music." And that's just what you get.

The circumstances of the tour were clearly very different from the 1970s. Then, CSNY could be confident that every last member of their audience agreed with them about Kent State (and the killing by National Guardsmen of four student protesters, whom the band honour in their era-defining song, Ohio) and the Vietnam War. Now, the tension is often palpable as they play to more ambivalent crowds.

"There's no movement," Young elaborates. "I'm still part of the movement but the movement has stopped moving."

Time, alas, has not stopped moving. The years have not always been kind to Young's fellow band members. Graham Nash, still trim and well turned out, has that orange skin and distorted vowels that Englishmen acquire after too long in California. But Crosby, shaped like an enormous pear with wispy white hair on top, looks like a living Doonesbury cartoon. And although Stills remains recognisable he, too, now comes in XXL.

Young has enough energy for all four of them. He abandons the staid, chair-based delivery of his Heart of Gold concert film - released here in 2006 - to charge about the stage like a demented water buffalo, head down and guitar pointed at the floor. As at his recent Hammersmith shows, he plays as though the instrument needs to be bullied into beauty.

He is unapologetic. "This type of music doesn't go with anything else," he says. "You can't mix and match. If you mean it, you're thoroughly consumed."