Can a 14 year old boy be the mahram for his sister/ mother on a journey?

Answer

For being a valid mahram of a female on a journey, the requirement is not restricted to age. Even if the boy becomes baaligh, it does not mean that he automatically qualifies to be a valid mahram for a journey.

The mahram must be able to assert control over the female whom he accompanies on the journey. She will be totally in the background, and he must be able to attend to everything along the journey. She must not instruct him in the manner he should act on the journey. On the contrary, he must have sufficient ability and authority to issue instructions and to veto any of her whims and fancies.