Men lacking sexual desire are like Mahram men in looking at women
31-8-2015 | IslamWeb
Question:
I am an eighteen- year-old Muslim girl and my aunt's husband is an old man who is like my father. He sometimes stays at our house for several weeks. Is it permissible for me to wear trousers in front of him? My aunt's son is twelve years old; should I wear the Hijaab in front of him?
Answer:
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.
If your aunt's husband is an old man who no longer has sexual desire for women, then there is nothing wrong with revealing in front of him what is allowable for you to reveal of your body in front of your Mahram (permanently unmarriageable kin) men. If this is not the case, then it is impermissible for you to do so because there are religious texts, including verses of the Quran and Ahaadeeth, which jointly prohibit men from gazing at women who are their non-Mahrams. Ibn Qudaamah 
said:
"As for men who lack sexual desire for women due to old age, sexual impotence, a chronic illness whose recovery is not expected, or due to being an effeminate old man who has no sexual desire for women, in all these cases, such a man is legally like the Mahram concerning the gaze. Allaah, the Exalted, says (what means): {... or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women...} [Quran 24:31] The verse refers to men who no longer have a sexual desire for women." [Al-Mughni]
We would like to warn the questioner that it is impermissible for her to wear trousers in front of any man other than her husband if the trousers are tight and outline and define the shape of her body.
As for the young child who is not yet sexually inclined to women, he should be treated like that old man. In this case, there is no harm for you to take off your Hijab in front of him. However, if he has begun to be sexually inclined to women, then you should observe the regulations clarified above.
Allaah Knows best.