I’m currently in 9th grade, and planning to start this summer because in 11th grade I have to take both. Should I start earlier?
Summer after sophomore year and take the exams in the fall of junior year.
There is no reason to start prep sooner than this summer, unless you want to take the exam this summer. For example, there are a few questions on the ACT that relate to geometry and possibly trig. My son took geometry and Algebra II his freshman year so took a practice ACT test at the local library his freshman year than took the ACT the summer before his sophomore year. He wanted to take it when both were still fresh. It worked for him. For my daughter it would not have worked. And for most summer after sophomore year to prep is ideal.
If you want to gauge where you are without taking the actual exam, you can see if your local library offers a practice exam for free. Or you could take practice exams at home with the Real ACT book or similar. If you do this, I would recommend simulating the real test conditions, meaning timing yourself with no distractions. Then you can decide if you want to start earlier.
It takes time to improve critical reading and writing. So you can “prepare” as early as possible by reading books, articles to improve your reading skills and vocabulary. That is the least painful way to prepare. People with good reading skills don’t need to prepare much at the time they take the test.
For SAT subject tests, you take them after you finish the related classes so that you won’t forget what you learn.