There is a well-know Shach in hilchos talmud torah who paskens that one can start learning kabbolo from age 40 on. The Vilna Goen, however, disagrees and claims that there is no such rule.
Practically how do we pasken? What are the prerequisites for learning chochmas hanistar?
In our day and age with the proliferation of seforim dealing with and explaining Kabbala on many levels, even to the uninitiated. It has become a norm to study these works, which provide many people with inspiration and insight. As long as this study does not replace the study of Shas and Poskim, and is not being done on the philosophical level of trying to figure out the essence of Hashem etc. this would be permitted even at a younger age.