David Laws, education minister, has also said that academies and free schools should be required to teach sex and relationship education.
Currently, sex education is compulsory in council-run secondary schools, for children aged 11 years.
Mr Laws said the idea to teach primary school children sex education would be a manifesto commitment.
"We have long made the case, both inside and outside government, for updated sex and relationships education to be taught in all schools, including academies and free schools, but it is not something the Conservatives are open to," Mr Laws said.
"We believe that by educating children about sex and relationships in an appropriate way, we can help them to make informed choices in their personal lives.
"Currently, academies and free schools have no requirement to teach sex and relationship education, depriving children of important life lessons.
"The Liberal Democrats are committed to ensuring that children have access to age-appropriate sex and relationship education regardless of where they go to school."
The Guardian reports that the Lib Dem plan would involve primary school pupils taught 'age-appropriate' sex education from seven years old – key stage 2.
Mr Laws added that personal, social and health education (PSHE), which forms sex education classes, should include citizenship and financial literacy, calling it 'curriculum for life'.
"By learning how to manage money and be a good citizen, we will be equipping children with the skills necessary to deal with the many practical issues that they will face in adult life, such as balancing a budget or voting for the first time," he said.
"Teaching these important life lessons will help to build the stronger economy and fairer society that Liberal Democrats want to see."
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