It's not about age; it's about having the wisdom necessary to be a witch. For instance, a witch has a responsibility to know how to use energy so that no-one (or herself) is harmed in any way. She needs to learn that anything she puts out into the world in the way of magickal work has the potential to come right back at her. She has to learn that she can't manipulate people's feelings and emotions. Or to work love spells that bind another person to her because she'll be the one that will end up being trapped.
Being a witch is a lifetime study, so I would say that it's never too early to start learning, but restraint in working with energy is necessary until you have the life experience and wisdom to know how to do it properly.