Only about one in five people under the age of 30 voted in the mid-term elections Tuesday, says a study based on exit polls.

The poor turnout among youth likely had some effect on the outcome of most races, but nowhere was this more dramatically highlighted than in the California ballot initiative to legalize marijuana. Political observers on Wednesday said the poor youth turnout in California accounted for the defeat of Proposition 19.

"Pot legalization defeated thanks to the elderly," reads the headline of a Justin Elliott article at Elliott points to a report that while six in 10 youth voters supported the measure, it was opposed by seven in 10 senior citizens.

Around one million fewer Americans under the age of 30 cast ballots in the 2010 mid-term elections compared to the 2006 vote, a study from the Center for Information and Research on Civic Learning and Engagement (CIRCLE) found. An estimated nine million people aged between 18 and 29, or one in five young Americans, voted Tuesday.

Young people are considered a key Democratic Party voter bloc, and their vote was instrumental in getting President Barack Obama elected two years ago, when a record 23 million under-30s cast ballots in the presidential polls.

In the last mid-terms in 2006, nearly 10 million young Americans, or around a quarter of under-30s, voted, said the study.

The youth vote in mid-term elections has held steady at around 20 percent since 1998, with a peak at around 23 percent in 2006.

The under-30s were the only age group this time around in which a majority favored the Democratic candidate over the Republican candidate, the study said.

A majority of young voters -- 56 percent -- voted for Democratic candidates in House races, and 40 percent for Republican candidates. The pollsters only had exit poll data from House races to work with.

Turnout this year among young voters was higher in states with a keenly contested vote and in traditionally Republican states than it was in states that tend to vote for the Democratic Party, the study found.

In "blue", or Democratic Party states, turnout was around 18 percent; in "red" or Republican states, it was 22 percent, and in so-called "purple" states where candidates from both parties were locked in a tough fight for the House, it was 23 percent, the study found.

"One explanation for the higher rates of participation in the purple states is that there was greater voter outreach and political advertising in these states," the study said.

The young voter turnout figures are estimates based on exit polls, the number of ballots counted and US Census data.

-- With a report from AFP