Hii Koushik!
The Eligibility criteria for the WBCS 2020 entrance examination states that the candidates must be from the age of 21 to 36 years of age, for general category and a relaxation of 5 years for reserved categories. The candidates must have a graduation degree from a reknowned institution and must know, how to speak and write Bengali except for Nepali candidates. You can refer the following link for more details -
https://www.google.com/amp/s/competition.careers360.com/articles/wbcs/amp
Hope this helps.
Good luck!