Police recorded 50 cases of children complaining of abuse by under-18s in a year across the two counties.
One child protection expert has said that young people's minds are being "warped" by easy access to hardcore pornography.
But the National Society for the Prevention of Cruelty to Children said its service in the city was helping to turn around harmful sexual behaviour in children and support victims of abuse.
The charity issued Freedom of Information requests to forces across the country.
Devon and Cornwall Police recorded 50 complaints of under-18s committing sexual offences against other children in the financial year 2013 to 14.
Their youngest perpetrator was aged 11 and the youngest victim just four years old, with crimes including serious sexual assaults and rape.
The figure is up from 43 cases over the previous 12 months.
The charity said that most victims knew the suspect, often with teenage boys offending against female acquaintances.
But the NSPCC said its local service "Turn the Page", was showing great success in getting offenders change their behaviour.
The charity's service manager Jacky Moon said: "It's deeply concerning that thousands of children are committing sexual offences including serious assaults and rape.
"For young children in particular we have to question the environment in which they are growing up that has led to them behaving in this way.
"Prevention has to be the key and that is recognising warning signs early and taking swift action. It could be that they have seen sexual activity that they are just too young to understand and are copying what they have seen.
"We also know that pornography for many older children is now part of life. Easy access to hard core, degrading and often violent videos on the internet are warping views of what is normal or acceptable behaviour."
Any adult worried about a child or in need of help and advice can contact the NSPCC's helpline on 0808 800 5000. Children and young people can contact ChildLine on 0800 1111.
