ANSWER:

He must reach the age of puberty or must reach 15 years, whichever comes first. 

https://islamqa.info/en/answers/197392/if-any-of-the-signs-of-puberty-appear-in-a-boy-he-becomes-accountable

What should be the the age of a son or a brother as to be a Mahram(irrespective of his maturity) so that a mother/ sister can travel with him .

Mahram | August 28, 2019 9:49 am
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I have a question about mixing with my aunts where I grew up with. They are much older than me (around 40 years). They are the wives of my uncles. Now that I am 22 years old is it permissible to sit with them if my uncle is home. And what about mixing with old women.