Response: The youngest age at which a male can become a mahram for a woman is the coming of age; And that (occurs after) the completion of 15 years (of age) or seminal discharge with desire or the growth of hair around the pubic region.
And when (any) of these three signs are present, then he becomes religiously responsible, and it is permissible for him to be a mahram for a woman. And likewise with the presence of (any) one of the three (signs) a woman becomes religiously responsible; And the woman has another sign and that is the (coming on of the) menses.
And Allaah is the Expounder of all success.
[By: Shaykh Ibn Baaz|Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah – Volume 16, Page 382]