Hi

You don't need to worry at all dear even if you are 21 years old

This is because,

There's no age limit for jee mains, so even if you are 21 years old you are still eligible for JEE Mains.

For example,

Eligibility criteria says that 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,  mathematics and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

So, if you satisy this criteria, then you are eligible for JEE mains 2021 even if you are 21 years old

thank you