More than 2,000 children have received treatment for eating disorders in the past three years, according to NHS figures.
Statistics show that nearly 600 youngsters under the age of 13 were treated in hospital in England, including 197 aged between five and nine, after developing eating disorders.
The figures from 35 NHS hospitals showed 98 were aged between five and seven at the time of treatment and 99 aged eight or nine. Almost 400 were between the ages of 10 and 12, with more than 1,500 between 13 and 15 years old.
The figures, released under the Freedom of Information Act, are believed to be an underestimate, according to reports.
Some NHS hospitals treating such patients refused to provide any data while others would only release figures for children admitted after becoming dangerously thin, excluding those undergoing psychiatric therapy as outpatients.
The findings come after experts called earlier this year for urgent action to improve the detection of eating disorders in children. About three in every 100,000 children under 13 in the UK and Ireland have some sort of eating disorder, according to a study conducted by experts from University College London's Institute for Child Health.
Figures released last October showed one in three hospital admissions for eating disorders involved a child, with under-18s accounting for 882 out of 2,579 admissions to England's hospitals in the 12 months to June last year.
Susan Ringwood, chief executive of the eating disorders charity B-eat, told the Sunday Telegraph that the figures reflect "alarming" trends in society, with young children "internalising" messages from celebrity magazines which idealise the thinnest figures.
"A number of factors combine to trigger eating disorders. Biology and genetics play a large part in their development, but so do cultural pressures, and body image seems to be influencing younger children much more over the past decade," she said.
A Department of Health spokeswoman said: "We are committed to improving mental health among the whole population. That is why we are providing around £400 million over the next four years to expand psychological therapies, including a specific programme for children and young people. Early intervention is essential for those with eating disorders."