Pregnancy History

Women who haven't had a full-term pregnancy or have their first child after age 30 have a higher risk of breast cancer compared to women who gave birth before age 30.
When breast cells are made in adolescence, they are immature and very active until your first full-term pregnancy. The immature breast cells respond to the hormone estrogen as well as hormone-disrupting chemicals in products. Your first full-term pregnancy makes the breast cells fully mature and grow in a more regular way. This is the main reason why pregnancy helps protect against breast cancer. Being pregnant also reduces your total number of lifetime menstrual cycles -- which may be another reason why earlier pregnancy seems to offer a protective effect.
Adopting children doesn't affect breast cancer risk.

Steps you can take

The decision to have a child is very personal, complicated, and requires commitment and support. The inability to have children can be very painful and the prospect of having a child without a partner may be daunting for some women.
Some women may choose not to have children. Others may choose to wait until they are older to have children. Many women who would like to get pregnant are stopped by infertility. After a breast cancer diagnosis, the opportunity to have children can be hindered by lingering treatment side effects (including infertility), and taking hormonal therapy medicine to reduce the risk of the cancer coming back (it's unsafe to take hormonal therapy while you're pregnant). In addition, research has not been done specifically to find out how pregnancy hormones affect risk of recurrence for women who've had hormone-receptor positive breast cancer. If you fit into this category and want to get pregnant, talk to your doctor and weigh any risks.
If having children earlier rather than later is an option for you, you may want to do that. Still, this is a highly individual decision affected by many factors besides breast cancer risk