A woman can perform Hajj without a mahram as long as she is traveling in the company of a trustworthy group, and her safety is guaranteed.
In his response to this issue, Dr. Salah Sultan, states,
It is permissible for a woman who is even younger than fifty-five to travel with a trustworthy group or when the way is safe. This is according to the opinion of Imam Shafi`i and Malik.
Some other scholars say it is not allowed, and they base their view on different interpretations of religious texts on this issue.
Lately, the Saudi government concluded that a woman over forty-five is allowed to perform Hajj and `Umrah without a mahram as long as she is traveling with a safe group.