The beginning of your junior year is an ideal time to begin preparing for the SAT and/or the ACT (if you haven't already started). So much of successful test prep is about stress reduction; students who have mastered the material and know the ins and outs of the test itself are ideally positioned to walk into their tests with the confidence to hit a maximum score. A window of at least 6-9 months to prepare allows us, as tutors, to help students reach that point in the least stressful manner possible. It gives us plenty of time to administer our proctored practice tests, a key component of any effective test preparation, and allows for many more options for SAT and ACT test administrations to choose from, so we can base decisions about test dates around student preparedness, and not looming deadlines.
This last factor is, perhaps, the single most important reason to start early. While I hate for any student to take a test more times than absolutely necessary, I always recommend that my students plan to take the SAT or ACT (whichever is the better fit) twice. There's no better feeling than walking into the first test administration knowing that no matter how it goes, you'll have another shot.
No matter how well they do, I find most students want to avail themselves of that second administration; some even want to take the test a third time, and starting early leaves them time to do so. Students who wait to prepare, however, may find themselves without that option, and will certainly feel more time pressure, which increases their stress level (and could easily hurt their scores). Furthermore, many students need to take SAT Subject Tests; starting early permits us to plan for Subject Test administrations that will enable students to take all the tests they need while spreading out the work as much as possible.
As Noodle Tutors, we understand that test scores are just one component of a college application, that students need a little room to have a life outside of preparing for college, and that while both are important, academic performance needs to take precedence over test prep. When we have more time to work with our students, we can be more flexible and avoid undue interference with academics, or overloading an already stressed high school junior. While no high school student relishes standardized tests (the possible anomaly being future tutors), given the right time frame, we can make preparing for standardized tests as painless as possible.
You also never know: you might even find you're among the lucky ones who prepares quickly and finishes testing mid-way through your junior year. Now, wouldn't that be nice!