hi,

as per 2020 upsc civil service exam prelims notifications, A candidate must have attained the age of 21 years and must not have attained the age of 32 years on the 1st of August, 2020 i.e., he must have been born not earlier than 2nd August, 1988 and not later than 1st August, 1999. so as for 2021 exam, 1st august of 2021, you should be 21 years of old . as per your date of birth you would be eligible to apply for upsc civil service mains on 2021. age relaxation is applicable for reserve category candidates.

refer to the below link to know about the details notification of 2020 upsc civil service  exams:

https://www.upsc.gov.in/sites/default/files/Notification-CSPE_2020_N_Engl.pdf