The lower age limit of the RBI GRADE B examination is 21 years and you should attain this age as of 1st January, of the appear year. Now as you aren't become 21 before 1st January,2023 you aren't eligible for RBI GRADE B examination. So you are eligible for your 1st attempt on 2023.

For more information regarding RBI GRADE B consider the link below:

https://competition.careers360.com/articles/rbi-grade-b-eligibility-criteria#:~:text=The%20age%20limit%20to%20apply,is%2021%20to%2030%20years.

I hope this helps. Wish you a great eve!