The minimum age requirement as per the UPSC CSE examination is that an aspirant must be of at least 21 years of age on or before August,1 of the appearing year. Now for the year 2024, you are turning 21 after 1st August,2024. So you aren't eligible for the UPSC CSE 2024. You are eligible for the examination from 2025 and onwards.

For more information regarding UPSC CSE consider the link below:

https://competition.careers360.com/articles/upsc-ias-eligibility-criteria

I hope this helps. Wish you a great day!