It will be a huge burden on you, if you start learning 11th and 12th portions now.
But, NEET preparation is not just 11th and 12th portions. You shall revise the 6th to 9th science portions now, which will help you in understanding the portions in the NEET syllabus.
It is not too early to start preparing. You shall start it, but be careful with portions that you study.