Answered by Shaykh Amjad Rasheed
You mentioned that when a girl reaches seven years of age, it becomes unlawful for a man to be alone with her. Does this mean that it is permissible for a woman to travel with her [h: male] mahram when he reaches seven years of age?
It is permissible for a woman to travel with a [h: male] pubescent or adolescent (Ar. murahiq) mahram who is competent [m: enough] such that she is safe with him. An adolescent is someone who is close to reaching puberty with respect to its average age, which is close to fifteen [m: lunar] years (Tuhfa, 7.197). A seven year old boy, therefore, would not fulfill this criteria. Allah knows best.
Amjad Rasheed
Amman, Jordan
(Translated by Moustafa Elqabbany and Hamza Karamali)