Hello student you should appear in NID entrance examination just after or in appearing year of class 12. Genral and EWS category can appear in NID entrance examination upto 20 year of age.

Eligibility - candidates must passed or appearing class 12 in any stream from a recognized board.

For more updates refer to the link mentioned below - https://www.google.com/amp/s/design.careers360.com/articles/nid-dat-eligibility-criteria/amp

Feel free to comment if you have any doubt

Good luck