The LibDem Chief Secretary to the Treasury said the issue facing voters in the referendum would not be the Scotland of 2014 but the Scotland of the future.
His comments came as experts told MPs that Scotland leaving the UK may trigger passport controls on the border with England and that the remainder of the UK may have to remove the colour blue from its flag.
There were also warnings from Labour that an independent Scotland might lose TV programmes like Strictly Come Dancing and accusations the SNP was attempting to be both "player and referee" of the referendum.
The SNP hit back at Mr Alexander, accusing him of "hypocrisy".
Earlier, he had warned forecasts suggested the Scottish population would age more rapidly than the UK as a whole.
He said: "The oil reserves are volatile and running out but the ageing population profile is about a generation ahead of the rest of the UK. There will be more older people in Scotland who will have to be supported by fewer people in work.
"I take difficult decisions on spending every day at the Treasury but the Finance Minister of an independent Scotland, should it come to that, faces making difficult decisions for decades."
His office produced figures showing that for every 100 people of working age in Scotland, there are currently 25.8 people aged over 65. But the figures state that will rise to 51.7 for every 100 people of working age by 2060. The figure for the rest of the UK would be 45.9.
Mr Alexander said it was startling that by 2060 there would be just 1.9 working Scots for every person over 65, compared to 2.2 in the rest of the UK.
He told members of the Lords: "As a United Kingdom of greater scale, greater diversity and broader fiscal base, we are much better placed to meet today's economic challenges and achieve prosperity tomorrow together. "We would all be weaker apart."
A spokesman for Deputy First Minister Nicola Sturgeon said: "This is the height of hypocrisy from Danny Alexander. Only an independent Scotland will be able to properly protect the gains we have made for older people in Scotland, such as free personal care and concessionary travel.
"The Westminster parties, such as the LibDems, are focused on axing these benefits."
Mr Alexander's comments came as Westminster held a series of debates and evidence sessions on Scottish independence.
Nigel White, professor of Public International Law at the University of Nottingham, told MPs the UK might have to change the union flag.
Asked if the rest of the UK would have to ask Scotland's permission to keep the blue in the flag, he said: "Formally, legally speaking if you see it as a case of continuation, as I do, then not.
"But politically speaking and practically speaking, these things need to be negotiated because you don't want to end up like Greece and Macedonia, forever arguing about a flag."
Meanwhile, Labour MP Anas Sarwar said the SNP was "living in a fantasy land" to suggest the current range of BBC services would be available in an independent Scotland.
Alistair Darling also accused Alex Salmond of attempting to be both "player and referee" after the FM said the question wording would be informed by the Scottish Government's consultation, as well as Electoral Commission testing.
Ex-Chancellor Darling, the chairman of the Better Together campaign, said: "Alex Salmond has started to give indications that when it comes to setting the rules, he wants to be the referee and a player on the same pitch. This is totally wrong."
The Scottish Government said it was the same processs used by the UK Government.