The voting age would be lowered to 16 as soon as 2016 under Labour proposals, Ed Miliband has said.
The Opposition leader has pledged to give 1.5 million 16 and 17-year-olds the right to participate in UK elections if his party wins the General Election in May.
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Mr Miliband set out the timetable as he became the latest politician to face questions from a young audience at a Leaders Live event – organised by voter engagement group Bite the Ballot and streamed by YouTube.
“Britain will only succeed as a country if we give our young people the chance to fulfil their potential and play their part,” he said.
“And when decisions are being taken which affect their future, a democratic country like ours should ensure that they have their voice heard.
“Too many young people are turning their backs on politics which is bad for our country and bad for them, too.
“That’s because too often young people don’t get a look-in with politicians who know they can’t vote – or assume that they won’t vote.
“The measures we will introduce in our first year of government represent the greatest extension of the franchise in my lifetime.”
The Labour leader wants the rules changed in time for the 2016 elections to the Scottish Parliament – though electoral law is one area set to be devolved to Scots.
There are also elections due to the Welsh and Northern Irish assemblies and Labour said it would work with both to make the change.
Other elections in May 2016 include those for the London Mayor and Assembly and in many local authorities.
Mr Miliband said schools and colleges would be required to help reverse a slump in the numbers of young people registered to vote.
This could include schools registering entire year groups as a block.
“It is not enough just to give young people the right to vote,” Mr Miliband said. “We must do everything we can to ensure they have the chance to exercise it.”