Sixteen and 17-year-olds should be barred from voting in a referendum on independence for Scotland, Scottish Secretary Michael Moore has said.
He also stressed the UK Government favoured a straight yes or no question, with no third "devo-max" option on ballot papers, and pressed the case for an early poll to bring certainty north of the border.
Scottish First Minister Alex Salmond wants 16 and 17-year-olds to have their say on the "country's future", but UK ministers want those under 18 banned from voting.
"So far we have not yet heard a convincing argument that says you should change the basis of the franchise on this one-off referendum, otherwise you could start opening the whole debate up to lots of other people saying they should be part of this or certain people shouldn't be part of it," Mr Moore told BBC1's Sunday Politics.
"I think the fairest basis is to use the franchise that elected the very parliament that gave the First Minister his mandate to have a referendum."
Mr Moore demanded "a simple, clear franchise which is the same as the Scottish Parliament one" - which would exclude anyone under 18.
But he said UK ministers would listen to the outcome of an ongoing consultation on how the referendum should run. He also dismissed claims ballot papers should offer voters a choice on greater powers over tax for Holyrood.
He said: "I don't think you can muddle up a question about whether Scotland stays in the UK - the most successful combination of nations in history - or goes its own way with a decision about more powers for Scotland within the UK."
The Liberal Democrat minister also said the British Government wanted a referendum earlier than Mr Salmond's favoured 2014 date.
Mr Moore added: "It's hard for us to see why on earth you would want to delay a decision as fundamental as this for another maybe three years. Let's not do that, let's get on with it sooner rather than later."