Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
A good number of scholars support the view of the Maliki scholars that a woman may travel for hajj in a safe and trustworthy company of women, whereas for the Shafi`i scholars the safe company could be of men as well.
Answering your question, Dr. Muhammad Salama, PhD, Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu), states:
There are authentic prophetic hadiths that prohibit women from travelling without mahram. Muslim jurists unanimously extend this ruling of prohibition to the Muslim women’s travel for a voluntary hajj.
As for the obligatory hajj, Muslim jurists have different opinions. The Hanifites and Hanbalites maintain the same ruling stipulating that a Muslim woman has to have a husband or mahram in her company.
The Malikites, however, allow her to travel in a safe and trustworthy company of women, whereas for the Shafi`ites the safe company could be of men as well.
They quoted in support of their opinion the narration reported by Adi ibn Hatim that the Messenger of Allah (peace and blessings be upon him) asked him “Have you been to Hira?” Adi replied, “I haven’t been to it, but I was informed about it.” He said, “If you should live for a long time, you will certainly see that a lady in a howdah traveling from Hira will (safely reach Mecca and) perform the tawaf around the Kabah, fearing none but Allah.” [Al-Bukhari]
A good number of contemporary scholars support this second opinion; particularly with regard to aged women.
Almighty Allah knows best.