if your age is between 16.5  to 19.5yrs then you can go for nda exam and you can also apply after graduation too

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You can apply for NDA. The National Defence Academy commonly known as NDA is conducted twice a year by the Union Public Service Commission for recruitment into the Indian Military Academy, Indian Naval Academy and Indian Air Force Academy.

Follwoing is the Educational Qualification for NDA-

• For Army Wing of National Defence Academy: The candidates applying for admission into the Army wing of National Defence Academy should be Class 12 pass.

• For Air Force and Naval Wings of National Defence Academy and for the 10+2 Cadet Entry Scheme at the Indian Naval Academy: The candidates must have passed class 12th with Physics and Mathematics conducted by a State Education Board or a University.

• Candidates who are appearing in Class 12 or equivalent examination can also apply for this examination. Those appearing in the class 11 exam are not eligible.
  

To know about NDA Eligibility Criteria regarding Physical Standards, Visual Standards, CLick below link-

https://competition.careers360.com/articles/nda-eligibility-criteria

All the best.