If you are serious about studying aerospace engineering in India, start early yes even before finishing your high school. Aiming for a degree in Aerospace Engineering means you'll need to meet key eligibility criteria typically 10 + 2 with Physics Chemistry and Mathematics.
The core subjects (Physics Maths) are heavy in aerospace engineering. If you build a strong base now you'll find the first year of college easier.
Many entrance exams for aerospace engineering call for good mathematics and physics knowledge earlier practice gives you a big advantage.
With earlier preparation you can also explore aptitude reasoning and technical skills all of which help you perform better in exams and in the tougher years of the degree.
Can I start preparing after Class 12th Yes - you can begin after Class 12th and still succeed. But you'll have less time and more pressure because you'll be catching up on fundamentals that others started earlier.
Is it too early if I start in Class 10th No - starting in Class 10 is a plus. It gives you more time to explore your interest strengthen subjects and evaluate if aerospace engineering is the right fit.
Will starting early guarantee success Not automatically - the key is consistency quality of study and understanding concepts deeply. Starting early gives you the opportunity but effort still matters a lot.
aim to begin serious preparation around Class 11th in the science (PCM) stream ideally even earlier. Build your command over Maths and Physics. Work on problem solving and reasoning. Research and appear for the relevant engineering entrance exams (which you'll find detailed in the section on Top Aerospace Engineering Entrance Exams in India). This approach gives you the best shot at getting into a good aerospace engineering program.