Do you need to have SAT scores from the same year that you apply to college? Or are the scores good for longer? Say, for example, I took the SAT junior fall and was satisfied with my score. Could I submit that score to colleges even though I didn't earn it within the application year?
Technically, SAT scores never expire. You definitely shouldn't worry about the validity of your scores if you take the test your junior year and want to use those scores to apply to college. Certain institutions place restrictions on the length of time between taking the test and submitting an application, but it's usually around five years.
According to the College Board, "Official score reports sent to colleges five or more years after a test date are accompanied by a message explaining that they may be less valid predictors of college academic performance than more recent scores."
The scores can be sent no matter how long it's been since you took the test, but there's no guarantee that the institution where you're applying will accept them as valid.
To that point, it is important to keep in mind that the SAT has changed significantly this year. Composite scores are out of 1600 again, and the Writing section is lumped in with Reading for just one score out of 800. Most colleges probably won't accept SAT scores from the old version of the test after two or three years because they won't be comparable to those of other applicants who submit the new test. Almost all schools will accept old SAT scores for the high school classes of 2017 and 2018 (except Northwestern for the class of 2018), but after that you'll be expected to present scores from the new version of the test.