A twelve year old would have a difficult time digesting the world-view presented in this novel, which, in effect, would mean that a twelve year old would miss the point.
A bright youngster could certainly take in the plot and grasp much of the character material, yet there is an emphasis here on a special type of bitterness, one that is culturally delineated and which would seem to require an adult's perspective to appreciate.
It all depends on the child. As a teacher, I would not choose it as a subject of study. Certain themes are provocative by nature and destined for more mature readers. Of course, this taboo element is largely the reason for its 'attractiveness' to a young reading audience.
As a mother, I would not forbid the book, either. A reflective child could make the distinction between the writer's personal reflection and expression and his own. Parental guidance would help the child decide for himself what are 'appropriate' values and what are not.
Vonnegut's provocation is his charm; you see it again in 'Cat's Cradle.' If you are looking for an appropriate text for children by the same author, why not try his short story "Unready to Wear?" Kids love it!