The Church has developed a sex education pack entitled All That I Am designed for nine-year-olds in Birmingham, a city with areas where as many as one in 10 girls conceives before they are 18.
It covers issues such as menstruation and physical and emotional growth - but not contraception. A second programme for pupils aged 11 to 14 is now being developed.
Most local authority schools already give older juniors sex education lessons. The Church has decided to bring forward the age at which it teaches sex to reflect children's earlier sexual awareness and the pressures of society. Previously the topic was only studied by 12 and 13-year-olds as part of the science curriculum.
Father Joseph Quigley, director of religious education for the archdiocese of Birmingham, said: "Our research showed that Year 5 was a key time for pupils to reflect on their attitudes."
The pound;25,000 project, which is government-funded, was carried out in collaboration with Birmingham City Council's health education unit. It contains a video and lesson plans and was developed with the help of teenagers, who were surveyed about their experiences of sex education.
Father Quigley said the issue of contraception will continue to be dealt with as part of the science curriculum in secondary schools.
He added: "The origins of the project were the experiences of teaching primary and secondary pupils and the need to provide a curriculum which was authentic to the Church's teaching and acknowledges pupils' physical and emotional development.
"Within the teaching of the Church we talk about sexual relationships finding their proper meaning in marriage.
"Children's perceptions are being formed by social and cultural contexts, the magazines they look at and the programmes they watch.
"What we are attempting to do is provide children with the tools to question those magazine and television programmes."
There are 54 primary and 10 secondary Roman Catholic schools in the city.