All children in state-funded schools in England would receive sex and relationships education from the age of seven under Liberal Democrat plans.
The "age-appropriate" classes would form part of a "curriculum for life", including education about managing money and citizenship lessons.
The plan would mean that personal, social and health education (PSHE) is compulsory in all state-funded schools including academies and free schools.
A report published by Ofsted last year found that sex and relationships education needed to be improved in more than a third of schools and called for secondary school pupils to learn more about issues such as pornography, relationships, sexuality and staying safe, rather than just the ''mechanics'' of reproduction.
Sex and relationships education is compulsory in local authority-run state secondary schools, but not in academies or free schools.
The Lib Dem policy would also extend compulsory sex education lessons to key stage 2 - covering children aged between seven and 11.
The proposals will form part of the Lib Dem manifesto at the next general election.
S chools Minister David Laws said: "It is vitally important that children learn all the life skills they need when they are at school, and Liberal Democrats believe that this should include learning financial literacy, citizenship and age-appropriate sex and relationship education.
"We have long made the case, both inside and outside Government, for updated sex and relationship education to be taught in all schools, including academies and free schools, but it is not something the Conservatives are open to.
"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. Liberal Democrats are committed to ensuring that children have access to age-appropriate sex and relationship education regardless of where they go to school.
"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.
"Teaching these important life lessons will help to build the stronger economy and fairer society that Liberal Democrats want to see."