Like the accented farter said, critical thinking is a set of skills and the practice necessary to learn them that allows one to examine any given idea and judge the credibility, usefulness, and accuracy present (or lack thereof).

It also involves a minor degree of language and reading comprehension as an underlying prerequisite. You don't have to be more than basically literate, and close to fluency with a given language to make use of the skills. That's important because some people think that critical thinking can't be effectively taught before a given age (usually stated as early as tweens and as late as high school). But as long as the person can process language well enough to understand basic questions, you can lay the groundwork for it as early as kids start school anyway.