By Terrence Edwards and Natalie Thomas

ULAANBAATAR (Reuters) - In a stretch of open grassland surrounded by yurts, some of the thousands of Mongolians headbanging to the likes of heavy metal band "Purgatory Destroyers" at the country's biggest music festival were thinking about politics as much as partying.

Many young Mongolians, not much older than the wind-swept, land-locked democracy squeezed between autocratic China and Russia, are disillusioned with the slow economy and established political parties, and could play a decisive role in parliamentary elections on Wednesday.

More than half Mongolia's three million people are under 30 and grew up during a time of rapid change following a peaceful political revolution in 1990 that saw the Soviet system replaced by democracy and the influx of influences from Hollywood, hip hop and heavy metal.

"The quality of politicians is, I think, very bad," said Khishigdelger, a festivalgoer at the Playtime Music Festival. "So Mongolians need to do something different."

Turnout at the polls is expected to be at an all-time low, amid widespread perceptions that the older generation has hung on to power to further its own interest at a cost to the rest of the country.

Economic growth has fallen from 17.5 percent in 2011, the year before the Democratic Party took power, to the IMF's projected 0.4 percent for this year.

The resource-rich country has struggled to adapt as the market for coal and copper slumped on weaker demand from China, and some blame officials for creating mining disputes.

"Maybe that older generation should just die off," says Umbanyamba Unenkhuu, who is running for office as a member of the National Labour Party.

Sporting a beard and tattoos, Unenkhuu, 36, said he had taken time off from organizing heavy metal concerts and playing in a band to pursue elected office.

"You know some of those members of parliament, they've been in office for at least like the last four terms, right? And I mean what has changed? Nothing," the rocker-turned-candidate said.

"I think they should all step down and give way for the new generation."

Unenkhuu acknowledges he will be in a tough race as he faces off against the two largest parties - the Democratic Party and the main opposition Mongolian People's Party (MPP).

Polls suggest that voters, fed up from four straight years of slowing growth under the Democratic Party, are likely to award more seats to the MPP, which ruled Mongolia when it was a socialist one-party state and has held power most often since democratic reforms in 1990.

(Writing by Michael Martina; Editing by Nick Macfie)