Hi Sarita

Here is the eligibility criteria to take NIIT exam

• 60% marks or equivalent in Class X in five subjects (Compulsory subjects - English, Mathematics, Science and Social Studies. The fifth subject can be indicative as per applicant’s choice.). The board can be CBSE/ICSE/any other board in India or an equivalent board if from a country outside India.
• 60% marks in Class XII in five subjects (Compulsory subjects - Physics, Chemistry, Mathematics or Biology (PCM/PCB) & English. The fifth subject can be indicative as per applicant’s choice.). The board can be any recognized board in India or an equivalent board if from a country outside India. In case of ISC board if an applicant has taken four subjects, aggregate of four subjects will be considered.
• 60% marks in PCM in Class XII if opting for CSE or ECE streams, ≥60% marks in PCB or PCM or PCMB in Class XII if choosing BT and ≥60% marks in PCB in Class XII, if opting for Integrated MSc CS.
• Should have JEE Main/BITSAT/SAT/NEET/Any other State Engineering Entrance Exam’s rank/score otherwise applicant will be required to appear in the NIIT University Engineering Test (NUET).

Age criteria info is not available. Probably you can take NIIT exam of 2021 if you are atleast 17. Don't forget to check the eligibility criteria when exam is declared from their official website