Reference: Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah – Volume 16, Page 382
Question: What is the youngest age a male can be a mahram for a woman if she wanted to travel?
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.