Early menarche (the age when the first menstrual cycle started), and late menopause (the last menstrual cycle) are both responsible for an increased risk of developing breast cancer, the principle being the same, estrogen having more time to act on the breast. Over decades, the average age of menarche is shifting to early due to a variety of causes.
Pregnancy & Breast Cancer Risk:
A lady who has the first child after 30 years of age has an increased risk of developing breast cancer. And so is the risk for al day who has never borne a child.
It has been postulated that disorders which prolong the menstrual cycles MAY cause an increased risk of developing breast cancer. There are no studies, which have proven this, though.
A combination of the factors above may increase the risk further. For example, an early menarche (say 11 years of age), and a late first pregnancy (beyond 30 years of age) will allow estrogen to act on the breast for a significant number of years (say 19 years in this case), and increase the risk of breast cancer. On the other hand, a later menarche (say 14 years of age), and not so late pregnancy (say 26 years of age), may give estrogen about 10 yrs to act, and such a lady may not have an increased risk of breast cancer.