Aristocratic Heian women lived restricted and secluded lives, allowed to speak to men only when they were close relatives or household members. Murasaki's autobiographical poetry shows that she socialized with women but had limited contact with men other than her father and brother; she often exchanged poetry with women but never with men. Unlike most noblewomen of her status, however, she did not marry on reaching puberty; instead she stayed in her father's household until her mid-twenties or perhaps even to her early thirties.
Donald Keene in his essay The Tale of Genji says:
In 996 she violated custom by accompanying her father to his post in Echizen, apparently in order to avoid marriage with a second cousin, the governor of Chikuzen. The suitor, Fujiwara no Nobutaka, was in his middle forties, had several wives and concubines, and a number of children, the oldest a son of twenty-six. The difference in age and these family circumstances, more than the personality of the man, may have occasioned Murasaki Shikibu’s reluctance to marry him; however, life in the unfamiliar, depressing surroundings of Echizen seems to have changed her mind: in the spring of 998, before her father completed his term of office, she returned alone to the capital, and that autumn she and Nobutaka were married.
Her birth year is undetermined, but Wikipedia indicates 973 or 978. In that case she couldn't have remained unmarried into her thirties. So where did that number come from? Can we get more accurate information on that from scholarly works that examine her early poems and letters?
Also why did she marry her cousin Nobutaka if she was so reluctant to do so just a few years earlier? Any more information on her change of heart? What were the circumstances?