Does the obligation of Ḥajj drop for the woman who has no maḥram?
Q. For the woman who does not find herself a maḥram while she has not yet fulfilled the [Ḥajj] obligation; which of the two would be better for her: to hand over the responsibility to someone else or perform Ḥajj with her maternal or paternal aunt?
A. If the woman does not find a maḥram for herself then the Ḥajj is not obligatory upon her. This is because she is not legally able to do it, and the Ḥajj is not obligatory except for the one who is capable. It is impermissible for her to perform Ḥajj with her maternal or paternal aunt without a maḥram.
Ibn ʿUthaymīn, Majmūʿ Fatāwā wa Rasāʾil (209/21)
Ḥajj and ʿUmrah – The Legislated Age of the Maḥram
Q. When can a son become a maḥram for his mother; is it when he reaches maturity or when he able to discern?
A. He can become a maḥram when he reaches maturity and has [sound] intellect. However, whoever does not reach maturity cannot become a maḥram and the same goes for the one who has a deficiency in their intellect.
Ibn ʿUthaymīn, Majmūʿ Fatāwā wa Rasāʾil (221/21)