Yes you do take it your junior year of college. Make sure that you have completed the Pre med curriculum upon taking the test. Make sure you have high grades in your science classes and pass the MCAT. That betters your chances of getting accepted into medical school.
You can also take it after college as long as you are continuing to pursue significant activities after you graduated. Either way, it is important to prepare thoroughly and I would recommend looking at either online materials or a formal class.
If you just need a content review and not a formal class, I would recommend taking a look at the Related Link below- they have an outline that is modeled after AAMC's (the testmaker) topic lists. Also, they have a bunch of practice questions so you can gauge where you are (and it's 100% free, unlike those other expensive companies).
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No, it is not a required test if that's what you were wondering. However, you don't need a degree to take it. Most pre-med students take it in their junior year of college as you do need a bachelor's degree to apply to most American medical schools.