When do big girls become little women? No one knows for sure, because the transition is often invisible. All of a sudden bicycles are less interesting than the boy down the street.

This is the subject of a remarkable photography show that is coming to a close all too soon. Sally Mann has shot a series of 12-year-old girls in Lexington, Va., which is her home town. This is significant because her subjects are extremely relaxed in front of her camera -- more than willing to pose or simply go about their business.

Mann's platinum prints show off minute details in myriad shades of black, white and gray. They add up to a deeply felt psychological study of adolescence -- each girl is half child, half adult. To be 12 years old, we learn, feels like standing on the edge of a cliff, contemplating the next step.

Perhaps the most pessimistic shot is of a pensive girl leaning against the hood of a car on which she has scrawled a one-word message to the world: "doom." Also gloomy is Mann's portrait of an uncomfortably chubby 12-going-on-65-year-old who sits listlessly on a couch staring into the photographer's lens. Our eyes are immediately drawn to the large Calvin Klein signature across her shirt. As a child whose body does not conform to feminine stereotypes, she wears this fashionable logo like a scarlet letter.

"Beauty" and "sexuality" (or the lack thereof) are Mann's dominant themes. Her subjects, dressed in Mom's clothes, strike imitative poses. One gorgeous young girl in high heels and pearls balances seductively on a pedestal -- knowing all too well what her sexuality will be "worth." Moms seem to guard the threshold between childhood and adolescence, and, for the most part, they keep the gates locked. A sullen child dressed up in lace holds on to a doll, as her mother fusses with her blond ringlets; a close-up of mother and daughter smiling reveals the daughter's mouth full of braces.

The 12-year-olds who appear without their mothers are not only much more independent, but also able to experiment with their erotic feelings before the camera's eye. Mann carries this theme further with a shot of a young girl with a braid sliding down her neck; a tiny breast is revealed beneath her shirt. There is no exploitation here. Mann has magnificently managed to catch the first blossom of female sexuality. (The Martin Gallery, 3243 P St. NW, through Nov. 3.