A 12-year old, who has reached the formal operational stage, is perfectly capable of finding out what society he wishes to live in, the difference between good and bad, just and unjust, etc through logical reasoning. He is perfectly capable of pointing out what is wrong, and also to explain why he thinks it's wrong and what one can do to improve the situation. They understand fairness, that even though something does not directly improve your situation it can still be good.

Thus, the voting age should be 12.