Children as young as two are using social media, according to new research.
The study has led to fresh fears that more young people are becoming addicted to sites such as Facebook, Instagram and YouTube.
More than 60% of teachers said that they were worried about under-fives using social networks, not only because of exposure to inappropriate content but also how it may affect their communication skills.
Half said children between five and 10 they had dealt with has been exposed to unsuitable or harmful online content, rising to 78% among 11 to 15-year-olds.
The research was commissioned by the children’s charity Barnardo’s.
Javed Khan, chief executive of Barnardo’s, said: ‘Although the internet offers incredible opportunities to learn and play, it also carries serious new risks from cyberbullying to online grooming.
‘Our new report also calls for more research to help us understand the impact of social media on children’s mental health; high-quality education for children, parents and professionals; and a focus on well-being in every school.
‘Children today see the internet as a natural part of their world.
‘Our job as a society is to make sure children are protected online just as they are offline.’