What are the age and capacity requirements for a person to be considered a Mahram?

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It is stipulated that the Mahram must be mature (baligh) and of sound mind (aqil). A young boy who has not reached puberty cannot serve as a Mahram. This is because the purpose of having a Mahram is the protection of the woman, which can only be reliably provided by an adult of sound mind; thus, maturity and sanity are prerequisites.