More than 8,000 under-18s were accused of sexual offences against other children in the last two years, according to figures obtained by the NSPCC.
The children's charity contacted 42 police forces across England and Wales with a Freedom of Information request asking for the number of under-18s who had been accused, the youngest victim and the youngest accused. Of the police forces contacted, four did not respond.
The youngest child to be accused was aged just six, while the youngest victim had not yet reached their first birthday.
There were 8,077 children who were accused of sexual offences - 3,868 in 2012/13 and 4,209 a year later. Crimes included serious sexual assaults, rape and obscene publication offences.
NSPCC chief executive Peter Wanless described the results as "deeply concerning".
Most victims knew their alleged abuser, with some of the most common crimes being teenage boys abusing female acquaintances, according to the NSPCC.
It said most abusers were male though there was a small proportion of female abusers, and victims were male and female.
Wanless said: "It's deeply concerning that thousands of children are reported as committing sexual offences including serious assaults and rape. Prevention has to be the key and that is recognising warning signs early and taking swift action.
"We know that for many older children pornography is now part of life. Easy access to hardcore, degrading and often violent videos on the internet is warping young people's views of what is normal or acceptable behaviour.
"It is also feeding into 'sexting', where teenagers are creating and distributing their own videos and images that are illegal and have led to prison sentences.
"And for very young children, such as those of primary school age or younger, we have to question the environment in which they are growing up that has led to them behaving in this way. It could be that they have seen sexual activity that they are just too young to understand and are copying what they've seen."
He believes this behaviour can be turned around if caught early, stressing "these children are not beyond help".
Quick action including therapy could help prevent them becoming adult sex offenders, he suggested.
Wanless said: "Most importantly, their victims need support to overcome what has happened to them. Sexual offences, whether committed by another child or an adult, can have lifelong consequences."
The NSPCC suggests that parents can help keep younger children safe by teaching them the 'underwear rule' which states that the area covered by their underwear should never be touched by anyone else.
Older children should be taught about consent and ensuring they know they can speak out if anything makes them uncomfortable, the charity added.