Having your first pregnancy before 20 versus after 35 lowers your chance of breast cancer by 50%.

For each birth, breast cancer risk decreases by 7%.

For each year of breast feeding, breast cancer risks decreases by 4%

Source: http://www.cancer.gov/cancertopics/pdq/prevention/breast/hea...

The mechanism isn't entirely worked out, but the hypothesis is that immature breast cells are more likely to cause cancer, and that pregnancy and breastfeeding cause breast cells to mature into cells that are less likely to become cancerous.

This paper explores the mechanism more: http://erc.endocrinology-journals.org/content/14/4/907.full