In Western cultures, girls' self-esteem declines substantially during middle adolescence, with changes in body image proposed as a possible explanation. Body image develops in the context of sociocultural factors, such as unrealistic media images of female beauty. In a study of 136 U.K. girls aged 11–16, experimental exposure to either ultra-thin or average-size magazine models lowered body satisfaction and, consequently, self-esteem. Self-esteem was also lower among older than among younger girls. Structural equation modeling showed that this age trend was partially accounted for by a corresponding downward trend in body satisfaction; this, in turn, was fully accounted for by upward age trends in awareness and internalization of sociocultural attitudes toward appearance, and in social comparison with media models. Results support calls for early educational interventions to help girls to deconstruct advertising and media images.