Hello Aspirant,

Regarding the question you asked, I would like to clarify that you are in fact eligible to apply to The National Defence Academy (NDA) once you have finished your 12th grade.
In the official announcement, the Union Public Service Commission specifies the NDA qualifying requirements for 2022. Before completing the NDA test form, candidates should confirm their eligibility for the NDA 2022. The eligibility requirements for the NDA exam 2022 include a number of elements, including nationality, age restriction, educational background, marital status, and others.

You can take the coaching for one year but mind that there is no age relaxation.
Age Restrictions: 16.5 to 19.5 years for all categories; the NDA exam does not allow for age relaxation.
Make sure you don't exceed the age.

ALL THE BEST