In one case, they said a four-year-old was treated for the eating disorder.
The average age of children with the illness has dropped from 14-and-a-half years to 12 years since 2001, a nationwide study found.
The Australian Paediatric Surveillance Unit surveyed all paediatricians in Australia over a 12 month period to June 2003.
Adolescent specialist Dr Michael Cohen, who worked on the study, said doctors had treated eight-year-old children with anorexia and that he had once treated a four-year-old boy.
The report found that high achieving children from successful, middle class families were most vulnerable to the disorder.
"These are people that place inordinate pressure upon themselves," Dr Cohen said.
"It is a pattern to cope with this stress."
Anorexia was no longer only a reaction to media images of slim women, but everyday stresses felt by both genders, Cohen said.
The report said eating disorders ranked as the nation's third worst health problem for girls under 18, trailing obesity and asthma.
While girls remained worst affected, the number of boys succumbing to the disorder had risen in the past two years, Dr Cohen added.
About one in 250 girls under 18-years-old were affected and one in 1,000 boys.
The disorder put children at risk of permanent physical damage at a critical period in their growth, and could result in later infertility, stunted growth and brittle bones as well as death, he said.
The study was headed up by child psychiatrist Sloane Madden.
"There is a growing sophistication among young children now," said Mr Madden.
They describe feeling 'fat' and speak of a desire to look like models and actresses," he said.
The study coincides with reports of an eight-year-old girl who was being marketed as an adult model by her parents.
The report drew the ire of Australia's minister for Children and Youth Affairs, Larry Anthony, who said the photos of a heavily made-up girl were not 'appropriate'.