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-aged 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.

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.

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. Typically, the teens reported more strenuous exercise, while during adulthood walking was most common.