What is the age at which a child is considered a mahram according to Sharia, and is a four-year-old child considered a mahram that prevents khulwa (private seclusion), or is only an adult a mahram?

One must differentiate between a mahram for travel and a person who removes illicit seclusion (khalwa).

1. The mahram for a woman during travel: This is anyone whom she is permanently forbidden to marry due to kinship, suckling, or marriage. The Hanbalis stipulated that the mahram must be an adult and sane, whereas the majority of scholars consider a discerning adolescent sufficient if the woman is safe with him. This latter view is the most preponderant.

2. Removing illicit seclusion (khalwa): Illicit seclusion is removed by the presence of: The husband. The woman's mahram (such as her father or son). The man's mahram (such as his sister or paternal aunt). A discerning child from whom one would feel shy. One or more trustworthy women. One or more trustworthy men. As for a very young child from whom one would not feel shy, their presence is as if they were absent. It must be noted that the presence of one or more women removes khalwa, but it does not permit a woman to travel without a mahram.