Hope you are doing good.
Below I have mentioned the eligibility criteria for NDA 2021:
Should be an Indian national
The candidate should be an Unmarried male
Minimum education qualification required for NDA is 10+2 (Army), 10+2 with Physics and Maths (Air Force & Navy)
Candidates should fall in the age group of 16.5 and 19.5 years
So by the time you complete your B.Tech you would have crossed the maximum age and would not be an eligible candidate. So I would suggest you to try now itself.
Hope this helps