TL;DR: Either the House of Lords or the government will back down, or the government will push for an early referendum
The House of Lords passed an amendment which grants young people aged 16 and 17 the vote in the EU referendum, by 293 votes to 211, on Wednesday.
The government is strongly opposed to changing the voting age, meaning that the Lords' decision may significantly delay the timing of the referendum.
David Cameron has said his government will not back down, and the House of Lords seems to be the same way inclined.
Is this a stalemate?
If neither change tack, we can expect a delayed referendum on the EU: slightly embarrassing for a major policy promise for the government.
The government could invoke the Parliament Act, overruling the Lords amendment - however, this can only happen if the bill passes in the Commons and is rejected by the Lords for a second time. All this depends on whether a year has gone by since the bill’s first instance of a second reading - which was only the 9 June 2015.
If the Tories choose this method, this means the Parliament Act would be invoked in June 2016 and four months notice for a referendum would be given, resulting in the earliest referendum date of October 2016 (if all goes smoothly).
The government could compromise and permit 1.5 million 16 and 17 year olds a vote in the referendum on EU membership.
However, this means that all these young people would have to be added to the electoral register and, under the new rules, current 14 and 15 year olds could be eligible to vote if it is held as late as 31 December 2017.
The electoral register says all this could be done in less than a year.
A poll into under 18 voting intention towards a possible EU exit by the British Election Study in May showed that this age group are more in favour of remaining in the EU than the rest of the public...
... Which isn't great news for Brexiters.
FYI, this is how public opinion of over 18-year-olds on a British exit from the EU has changed since 2010: