Dakota Fanning will turn 13 next month, and she has a short answer for anyone who questions her decision to play a 1950s girl who gyrates in her underwear, wakes up as her naked father climbs into her bed, demands that a prepubescent boy expose himself to her in exchange for a kiss and, finally, is raped by a teenager who lures her with tickets to an Elvis concert: She’s growing up. Get used to it. Ms. Fanning is starring in “Hounddog,” an independent film that is to have its premiere on Monday at the Sundance Film Festival. It has already won attention far out of proportion to its budget of less than $4 million. David M. Halbfinger reports.