The Prime Minister has said the House of Commons will vote on whether to let 16 and 17 year olds vote in the referendum on leaving the European Union.
David Cameron re-iterated his position that the voting age should not be lowered from 18, but signalled it will be up to MPs from all parties to decide.
During Prime Minister's Questions, he was asked by Labour deputy leader Harriet Harman: "Why won't he let 16 and 17 year olds vote? This is about the future of our country. They did in the Scottish referendum. It's their future too."
Mr Cameron responded: "I believe this House should vote on the issue. The Conservative manifesto is clear, and my position is clear. I think we should stick with the current franchise of 18. But the House of Commons should vote."
With a majority of 12, Mr Cameron will hope he can see off the demand from Labour, the SNP and the Lib Dems - who have all said they want the voting age lowered for the EU vote - despite misgivings among some Tory backbenchers.
His comments represent a shift in position after Foreign Secretary Philip Hammond said yesterday the Government is "not persuaded" to lower the voting age to 16 in the referendum on whether the UK should stay in the European Union as he questioned how often young people take part in elections.
The minister suggested time would be best spent on "trying to get a decent rate of turn-out among 18 to 24 year olds".
His comments, which prompted an angry reaction from some MPs, came as legislation paving the way for an in-out vote by 2017 passed its second reading in the House of Commons.
Labour wants to get the Bill changed during the committee stage, as revealed by Shadow Foreign Secretary Hilary Benn on The Huffington Post UK.
Tory backbencher Sarah Wollaston said that "since nearly one in four 16 year olds can expect to live to 100, and they'll be living with the consequences of this far longer than members of this House" then "we should extend the franchise".
Alex Salmond, the SNP’s Foreign Affairs spokesman, said it was “deeply insulting” to under-18s that they were being denied the vote.
Labour's Mr Benn said Mr Hammond's comments have a “ring of familiarity” since they had been used to oppose giving women the vote in the past.
"On every occasion in the last 200 years, when someone has had the temerity to suggest the franchise should be extended, the forces of conservatism have said 'don't be ridiculous, it will undermine the fabric of society'," he said.
On votes for 16 and 17 year olds, which was adopted in last year's Scottish independence referendum, Mr Hammond said: "I concede that there are those in the House who will wish to debate whether that franchise in itself should be extended to 16 and 17 year olds. The Government is not persuaded."
When urged by Green MP Caroline Lucas to "let young people have a say on their futures", Mr Hammond said his personal view was that "we well be better expending our efforts trying to get a decent rate of turn-out among 18 to 24 year olds before we start worrying about 16 and 17 year olds".
Foreign Secretary Philip Hammond said time would be best spent on "trying to get a decent rate of turn-out among 18 to 24 year olds"
Labour MP Chukka Ummuna said the justification was "utterly pathetic and woeful".
The Bill could face trouble in the House of Lords where the Tories do not have a majority.
Lib Dem leader in the upper chamber, Lord Wallace of Tankerness, warned the Tories not to get resist giving 16- and 17-year-olds the vote.
The EU referendum will be the first of its kind since 1975.