How are critical thinking skills acquired?

Here on Fluther we all value those who don’t blindly follow whatever fad is going on now. But how is it some people can sift through the BS to find the truth, and others can’t (or won’t)? Is it a strictly learned process, and if so, how were we taught it? At what age does it begin? Can you give me some examples of an incident where you began to teach your child this skill?