For the NDA exam the age limit is generally 16.5 to 19.5 years, and only unmarried candidates can apply.
Both male and female candidates are eligible now, for the Army, Navy and Air Force wings.
Age is counted on the cut-off date given in the official UPSC NDA notification, so check it carefully.
There is no age relaxation for reserved categories in NDA, unlike other government exams.
You must have passed or be appearing in 12th with the subjects required for your chosen wing.