Is it permissible for a son to wash the back of his 75 year-old mother when there is no one to help her?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

According to most scholars, a woman's ‘Awrah (i.e. body parts that must be covered) before her Mahram men includes what is in between the chest and knees. Accordingly, none of her Mahram men are allowed to see or touch what is in this area, even if he is her son. Allaah The Almighty Commanded that the children who reached puberty must ask permission in order not to see their fathers or mothers' ‘Awrah, Saying (what means): {And when the children among you reach puberty, let them ask permission [at all times] as those before them have done.}[Quran 24: 59]

Commenting on this verse, Al-Qurtubi quoted Az-Zuhri as saying that a man must ask for permission before entering upon his mother and that this is the meaning for which this verse was revealed.

Seeking permission was prescribed to prevent looking at what is unlawful to look at.

Consequently, a woman is not allowed to unveil her ‘Awrah before her son and he is not allowed to wash her back. Being an old woman does not affect the Sharee‘ah ruling.