A resistance training program, while beneficial for all children and teens , is particularly important for young adolescent girls, says a 2014 international consensus position statement. (Loyd RS, et al. 2014)
Musculoskelatal growth during puberty, in the absence of corresponding neuromuscular adaptation, says the statement, may facilitate the development of abnormal joint mechanics such as increased knee valgus (e.g. knock knees) movement when landing, and injury risk factors. If not addressed, such risk factors may continue to develop throughout adolescence, the statement says, predisposing female athletes to increased risk of injuries.
Early participation by female athletes in a multifaceted resistence training program, says the statement, can:
The statement suggests that resistance training may be most effective for young adolescent female athletes, citing a recent literature review  by a one of the statement's co-authors, Dr. Gregory Myer of Cincinnatti Children's Hospital, which found, based on a review of 11 studies, that the earlier female youth can engage in a well-rounded training program including resistance training the lower the likelihood of ACL injury.
Loyd RS, Faigenbaum AD, Stone MH, et al. Position statement on youth resistance training: the 2014 International Consensus. Br J Sports Med 2014;48:498-505. doi:10.1136/bjsports-2013-092952
Myer GD, Sugimoto D, Thomas S, Hewett T. The Influence of Age on the Effectiveness of Neuromuscular Training to Reduce Anterior Cruciate Ligament Injury in Female Athletes. Am. J. Sports Med. 2012;20(10). DOI:10.1177/0363546512460637 (published online ahead of print October 8, 2012)(accessed October 15, 2012).