When do witches come of age?

The answer to this question depends largely on the tradition the witch follows, and the meaning you place on the term "Coming of Age".

In many pagan paths male witches "come of age" or are considered old enough to make their own decisions, when they have reached the age of 12 and have gone through a "Rite of Passage". Some Rites of Passage are as simple as "Tending the Yule Fires" and some require "Dream Quests" or physical tests depending as I said above on the tradition they follow.

In many pagan paths female witches "come of age" when their menses starts. There are usually "Rites of Passage" for the young women as well, all of varying degrees of difficulty.

The other "Coming of Age" for some witches has to do with them coming into their power, which depends on when they start their learning and how dedicated they are to that learning. Many study, not all ever progress to the point where they have true power.