Anant, For being eligible for JEE MAINS 2020, Candidates should have completed their 12th either in 2018 or 2019 or should be in their 12th in 2020 or should be completing their 12th in 2020. But for being eligible for JEE Mains 2021. Candidates should have completed their 12th either in 2019 or 2020 or should be in their 12th in 2021 or should be completing their 12th in 2021. So if you completed your 12th in 2018 and thereby qualified for JEE Mains in 2020, the same cannot be repeated for JEE Mains 2021. In other words, if you have completed your 12th in 2018 or before and then qualified for JEE Mains 2020, that eligibility cannot be applied to JEE Mains 2021. To be eligible for JEE Mains 2021. you should have completed your 12th in 2019 and by 2021. To qualify for JEE Mains, there is no minimum aggregate criteria. But if you want to join NITs, IIITs or CFTIs then you should score a minimum aggregate of 75% only in your 12th. 11th marks aren't considered. And then to appear for JEE Advanced which is the gateway for admission to IITs, you will need to fulfill the 75% aggregate condition. Now it is 75% for General Category and 65% for Reserved Categories. So how much you score in 12th is what matters to secure your eligibility for NITs, IIITs, CFTIs and JEE Advanced.

Hello

Weather student will be eligible for JEE mains 2021 or not depends upon his passing year.

As per eligibility criteria,  to be eligible  for JEE mains 2021 for BE or Btech you just must be either appearing for 12th board exam in 2021 or should have passed it in 2020 or 2019 with Physics,  mathematical  and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

So , if fulfill this criteria then yes you are eligible for JEE mains 2021

Thank you