Question: Can an 18 year old girl be with her grandfather with her hair uncovered?
Allah the Almighty commands:
“Tell the believing women to reduce some of their looks, and cover their private parts. Tell them not to uncover their beautiful parts except what is naturally seen. Tell them to put their scarves onto their chests and not expose their beautiful parts except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, their women, those in their possession, those male attendants that have no sexual desire to women or children who are not yet aware of the private aspects of women. (…)” (An-Noor / The Light 24:31)
According to this verse, a woman can be with the men above with her hair uncovered. Grandfathers are considered like fathers, so a woman can be with her grandfather with her hair uncovered.
Please see also the following: