An young child below the age of puberty can’t be mahram for his mother
The pre-conditions of a mahram
Is it possible for my son who is nine years of age to be my mahram, since i travelled with him; Is there anything wrong in that?
Regarding the mahram, the Scholars have mentioned that two conditions must be met:
1) The coming of age (puberty);
2) Mental awareness.
So, whoever has not yet attained puberty then it is not correct for him to be a mahram; and whoever is not yet mentally aware then it is not correct for him to be a mahram since the intent behind the (presence of a) mahram is protection of the wife and maintaining her and preventing any attack upon her. (Clearly) the young child is unable to undertake any of these (roles).
So I say, now the woman – as per the question – has reached her destination, so she is not to return to her husband until he comes and collects her and takes her (back) with him. Alternatively, she can travel with one of her mahrams (father, brother, etc.) – from amongst those who have attained puberty and are mentally aware. As for the young child who is nine years of age, then he cannot be her mahram.
Shaykh Ibn ‘Uthaymeen
I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 369
Youngest age a male can be a Mahram
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.
Shaykh Ibn Baaz
Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah – Volume 16, Page 382