The age limit to appear for National Defence Academy (NDA)entrance examination is 16.5 years to 19.5 years. And you will be eligible to appear for NDA 1 2021, NDA 2 2021 and NDA 1 2022. To check the complete eligibility criteria please refer the link that is attached
To be eligible to Indian Naval Academy and Indian Air Force Academy you must have passed 10+2 with Mathematics, Physics and Chemistry and for Indian Military Academy you must have passed 10+2 in any stream. The candidates who will appear in the qualifying examination in 2021 are also eligible to apply but their admission is subjected to the result obtained in the qualifying examination that is 10+2.