Dear student,

According to the basic eligibility criteria of NDA Exam, any student whose age lies between 16.5-19.5 years is eligible to give the NDA test. Now clearly, as your age is 18 years and 10 months, so you are eligible for this exam. Let me tell you that, this exam is conducted twice a year. Once in September and again in April . Now, at the time of April 2021, your age will be 19 years and 2 months. So, surely you can give NDA test in April. But for september term,  you are above the age criteria.

Hope this helps.