How old does a son have to be to be a mehram

wa`alaykum as-salam wa-rahmatullah

All praise is to Allah. Blessings and peace be upon the Messengerof Allah.

The mahram with whom a woman travels should be
1) Adult (baligh) by the definition of the shari`ah, (or at least near adult [Durr al-Mukhtar]).
(Adulthood for these purposes is defined as the first period for a girl, or the first wet-dream for a boy, or 15 lunar years of age for both as a precautionary upper limit.)
2) Sane
3) Upright, trustworthy and responsible

There is not a specific age which can be used universally; rather what is important is that the mahram (the son, in your case) fulfills the conditions mentioned above. The rationale is that the mahram should be capable of protecting the woman and her honor. Clearly a minor child and an insane person cannot even take care of themselves, let alone watch out for someone else. nor can an irresponsible or morally corrupt person be expected to fulfill this role.

[References:
- Radd al-Muhtar, Ibn `Abidin, 3/464 (Dar al-Kutub
al-`Ilmiyyah)
- al-Mughni, Ibn Qudamah, 3/193-4]

And Allah knows best.

-Suheil Laher