Hi there, 

As your age is 31 years you are not eligible to write the NEET examination. 

For unreserved category, the upper age limit for becoming eligible to write NEET is 25 years whereas for reserved category it is 30 years. 

For more relevant information please refer to the following link:

https://www.google.co.in/amp/s/medicine.careers360.com/articles/neet-eligibility-criteria/amp

Wishing you luck!