Adelaide clinical psychologist Rita Princi said girls and boys were being exposed to things such as sexy music videos, women's magazines and teenage concepts way too young.
This included teen-themed movies and TV shows such as High School Musical and Hannah Montana, huge among 6 to 8-year-olds, and suggestive music videos shown on weekend mornings.
Ms Princi said watching and reading things meant for an older audience had contributed to girls as young as six dieting and teen boys using steroids to build the "six pack" physique of male stars.
"They're . . . attracted to the way the girls are dressed or believe that's the way you're supposed to act around boys," she said.
"Girls as young as six years desire a thinner body and have awareness of dieting.
"We're seeing more boys wanting to have this perfect ideal body as well."
Ms Princi, whose practice is in Adelaide, will today tell a seminar by Kids Free 2B Kids and the Australian Council on Children and the Media that parents must say no when children want something they are not ready for.
She said research also showed parents played a big part in exposure to concepts which could damage their self-esteem, by commenting on their weight or not filtering inappropriate media.
Child-appropriate media like ABC Kids, however, did not cause such problems.
"The more exposed they are to the media and through their parents, the more at risk they were," she said.
"In particular, watching music videos and looking at magazines, especially if younger girls have access to their mother's or older sisters' magazines, (can lead to) lower self-esteem and poorer body image.
"The strong influence of parents' dieting behaviours and parental comments about their pre-adolescent children's bodies, especially for daughters . . . also places them at risk."
Melbourne University Centre for Applied Philosophy and Public Ethics researcher Dr Cordelia Fine will tell the seminar that older children are just as influenced by advertising as those under 11.
She said research showed older children, whom many assumed would be more savvy and sceptical, were still under the spell of marketers.
"This means that resisting the marketing involves a very different set of psychological tools than traditionally assumed," she said.
Rebecca, 8, has loved High School Musical and Hannah Montana for about two years.
She saw all three HSM movies, went to the HSM Ice Tour and watches Hannah Montana on TV. She also has T-shirts, DVDs, books, posters, pyjamas, a dance mat and blanket featuring either Hannah Montana or her HSM heroes. But she doesn't dress like them and shows no interest in boys - yet.
The Kids Free 2B Kids and the Australian Council on Children and the media seminar today will discuss the premature sexualisation of children.