Dear Khaja, for NEET 2019, the age eligibility criteria is that you should have completed 17 years atleast by December 31st, 2019 and there is no upper age limit. Going by the same rule and assuming NTA doesn't change the age criteria for being eligible for NEET 2020, you should be atleast 17 years old atleast by December 31st, 2020 and going by no upper age limit criteria, you will be eligible to appear for NEET 2020.