Hello Sandeep,

Candidates are required to fulfill the minimum eligibility criteria for appearing in the UPSC CSE examination. Candidates are required to have completed their graduation from any recognized university incorporated by an Act of the Central or State Legislature of India or established by an Act of Parliament or universities declared to be deemed as University under Section-3 of the University Grants Commission Act, 1956. Students in their final year can also apply for the examination. However, candidates are also expected to have a minimum age of 21 years by August 1 of the year they appear for the examination.

Therefore you're eligible to apply for the UPSC CSE Examination given that you're 21 years old or will be 21 years old by August 1, 2020.

Hope this helps you. We wish you the best for your future endeavours!

Regards