WASHINGTON - Get your daughters off the couch: New research shows exercise during the teen years -- starting as young as age 12 -- can help protect girls from breast cancer when they're grown.

Middle-age women have long been advised to get active to lower their risk of breast cancer after menopause.

What's new: That starting so young pays off, too.

"This really points to the benefit of sustained physical activity from adolescence through the adult years, to get the maximum benefit," said Dr. Graham Colditz of Washington University School of Medicine in St. Louis, the study's lead author.

Researchers tracked nearly 65,000 nurses ages 24 to 42 who enrolled in a major health study. They answered detailed questionnaires about their physical activity dating back to age 12. Within six years of enrolling, 550 were diagnosed with breast cancer before menopause. A quarter of all breast cancer is diagnosed at these younger ages, when it's typically more aggressive.

Women who were physically active as teens and young adults were 23 percent less likely to develop premenopausal breast cancer than women who grew up sedentary, researchers report today in the Journal of the National Cancer Institute.

The biggest impact was regular exercise from ages 12 to 22.

"This is not the extreme athlete," Colditz said.

The women at lowest risk reported doing 3 hours and 15 minutes of running or other vigorous activity a week -- or, for the less athletic, 13 hours a week of walking.

Why would it help? A big point of exercise in middle age and beyond is to keep off the pounds. After menopause, fat tissue is a chief source of estrogen.

In youth, however, the theory is that physical activity itself lowers estrogen levels. Studies of teen athletes show that very intense exercise can delay onset of menstrual cycles and cause irregular periods.

The moderate exercise reported in this study was nowhere near enough for those big changes. But it probably was enough to cause slight yet still helpful hormone changes, said Dr. Alpa Patel, a cancer prevention specialist at the American Cancer Society, who praised the new research.