He told a fringe meeting at the Labour conference in Brighton this week that there was a clear need for change.
The final report of the Tomlinson inquiry into 14-19 exams will put forward a four-level diploma, to replace A-levels and GCSEs, later this month. It aims to tackle England's high post-16 drop-out rates and give youngsters a clearer view of how qualifications can lead to work or further study.
A newspaper report this week quoted an unnamed minister confessing to "jitters" about such fundamental change and a senior government source arguing that the need for change had receded after this year's exams season passed smoothly.
But Mr Miliband said: "We have a unique chance to make changes at 14-19.
Every sector in society has a vested interest in getting this right. We have a chance to make changes which have eluded successive generations."
Charles Clarke, the Education Secretary, gave the Labour party conference his clearest indication yet that the Government was planning for major curriculum changes.
He said: "We have to move forward over the next Parliament to reform the curriculum for 14 to 19-year-olds. We have to address this agenda."
The report was the subject of one of the biggest rows of the conference, between Mike Tomlinson, the former chief inspector of schools leading the inquiry, and an academic who criticised the plans.
Professor Alan Smithers, director of the centre for education and employment research at Buckingham university, said the diploma did not fit the current system and would put young people off attending college. "Mike Tomlinson's trying to turn a deciduous tree into a conifer. It's not only going to be difficult, it's going to be ugly as well. He's barking up the wrong tree," said Professor Smithers.
Mr Tomlinson accused Professor Smithers of being obsessed with "bloody qualifications" and suggested he was out of touch.
Some of Mr Tomlinson's proposed reforms, including a new A** grade at A-level and the use of electronic "transcripts" of pupils' achievements, could be introduced within three years, the Times Higher Education Supplement reported this week.
Labour conference 14 15