NDA

Yes, you are eligible for NDA Exam. Candidate's age must be in between 16.5 to 19.5 years and appearing or passed in class 12th student can apply there. All the best.

At his current age of 18, he is eligible to apply for the exam.

Pathfinder for NDA
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Reference previous year solved paper by Arihant publications
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By Kumar Sundram.

If you have filled the Jan form, you can expect the admit card by the month of June.

Not yet. He may have to wait another year to be eligible. 17 years is the minimum age required.

Between the two, it would be ICSE. But, CBSE board is truly a good option, if you are seeking to attempt exams held by Centre. CBSE books (NCERT) form the basis for such exams and NDA exam paper is made from these books.

Yes, but you will have to attempt Maths paper in entrance exam and pass in that. You will only be eligible for Army Wing however.

Usually, it takes 4-5 months, and for the January form, you will get it by mid June.

Well, you are not eligible for NDA, the option left with you is CDS examination for which you need to do graduation. And after clearing that, you need to undergo training in AFA Hyedarabad. After which you become a pilot in Indian Air Force.

Medical examination is the last examination for selection in NDA and it includes parameters, which are complex and understood by doctors and specialists. Written test - SSB (5DAYS) - Medical test. So you can apply for the written test and then can appear for irrespective of your surgery. But in the end it all depends on the medical.