Is it a Prerequisite that a Mahram Has Reached the Age of Puberty?

Question posed to Sheikh Saalih Al-Fawzaan حفظه الله تعالى:

Q: Virtuous Sheikh, may Allaah grant you success, this questioner asks:

Is it a prerequisite for the mahram that he has reached the age of puberty and is sane during travels?

A: Yes, it is a must that he has reached the age of puberty and is sane, otherwise, he is not suitable to be a mahram during travels.

‏https://www.alfawzan.af.org.sa/ar/node/18352