Hiii

NIFT is a national level entrance exam for admission into fashion designing courses at National Institute of Fashion Technology.
There is always a particular eligibility criteria to apply for a particular course and for NIFT the Eligibility does not include the minimum age limit but it is clear that you must have to qualify 10+2 exam toh get admission into any UG course of fashion designing.

Since you are in class 11th you are not eligible to get admission, but for the purpose of an experience and practice you can apply for NIFT and if your application is not rejected then you can appear in the entrance exam but be sure on qualifying the exam you will not be able to hold you result for the next year. after completing your 12th class you will have to appear in the exam again to get admission into any course. 

I hope this helps. Feel free to revert back to us in case you have anymore doubts.

Thanks!!