As a rule of thumbs, the faces of old women are commonly less attractive ..., so, based on this theory:
What is the opinion of Islam regarding looking at old women (as non-mahram) without hijab? Is it haram or halal?
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The thing with old women is, by that age, they will have lost their beauty, or men don't feel that lust when they look at those old women the same and exact way they don't feel that lust when they look at their own mother or sisters.
So, the old woman should be old enough and have lost significant amount of her beauty.
To the question, Can men look at old women without veil (Hijab)?
No matter whether the old woman is covered or not with Hijab, looking at women with lust is not allowed, and hence the verse,
Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts, etc.). That is purer for them. Verily, Allah is All-Aware of what they do. (Qur'an 24:30)
Which justifies and gives a shortcut to avoid sin and lustful desires.
May the creator guide us all.
Bismillah (in the name of Allah),
Allah (SWT) says in Surat An-Nur, verse #60
والقواعد من النساء اللاتي لا يرجون نكاحا فليس عليهن جناح ان يضعن ثيابهن غير متبرجات بزينة وان يستعففن خير لهن والله سميع عليم
And women of post-menstrual age who have no desire for marriage - there is no blame upon them for putting aside their outer garments [but] not displaying adornment. But to modestly refrain [from that] is better for them. And Allah is Hearing and Knowing.
Putting aside their other garments is the only thing allowed for old women. However, old women must not uncover their hair in front of non-mahrams, as stated by the majority of mufasereen (commentators) regarding that verse. Their evidence is that the old woman cannot pray while her hair is uncovered, and the old woman is equal to the young woman in that regard, hence, she is not supposed to uncover her hair in front of non-mahrams.
Note: I did not go into details about other garments that was mentioned in the verse because I understood that it is not a concern. Your question was clearly about the hair only.
Allah knows best!