One in Four says there has been a marked increase in the number of young sex offenders.
The group works with survivors of abuse, as well as running treatment programmes for offenders.
In its annual report for 2013, One in Four revealed that almost a quarter of offenders in its programme are young men aged 18 to 25.
Executive director of One in Four, Maeve Lewis, says young people need to be taught about healthy sexual relationships.
She said: "Most of these young men began offending by downloading images of child abuse.
"Given the incredible access to all sorts of different devices young people now have to child abuse images, or to porn images generally, it raises a question for us (of) how are we going to support young people to develop a healthy sexual identity and to understand the importance of sexual intimacy within a consensual relationship."