Hey,

To be eligible for the NDA Exam , you must be either in 12th class or had completed the 12th class. And also, your age must lies between 16.5-19.5 years. Now as your age is 18.9 years, so clearly you are eligible for this exam.

But let me tell you that, you are not able to give the NDA 2022, exams, because at that time you will be above 19.5 years. But you can give the NDA 1 of 2021. You are eligible for that. Also, you can give either one of these exams or both of the exams. There will be no issue in that.

Hope it helps.