Early as possible you start, the more time you will have to prepare for NEET & JEE Competitive exams. Advantages of starting early preparation during class 9, 10, 11 and 12: You get plenty of time (2 years) to understand each concept, Many people think that in schools we are only taught derivations in physics, but I really think that if we do derivations along with practicing questions it actually makes us feel things, we can actually understand concepts better and we don’t need to actually do cramming of the formulas (I am not telling you to literally learn derivations but yes you hould have knowledge of each and every derivation mentioned in NCERT), You have a fresh mind to gather things better in mind, You don’t have pressure in your mind. The earlier you start the more questions that you can practice. The more you practice, the more confident you become with your preparations. And if you are confident in your preparation, then you will be able to attempt the exam also smoothly.