25 nov 1996 is my dob , i am eligible for cds 1 2021

There are different segments in CDS and the age limit is different for all of these. If you want to join Indian Military Academy or Indian Naval Academy then you must be within 19 to 24 years old and for Air Force Academy, your age must be within 20 to 24 years and for Officers Training Academy the candidates' age must be within 19 to 25 years.
Till now UPSC has not released the eligibility criteria of CDS 2021. According to the above age limit candidates born not earlier than January 2, 1996 only are eligible for CDS 2020 but as you are asking it for CDS 2021 so it will be increased by one year so therefore the date becomes January 2, 1997. So you will not be eligble according to this years' eligibility criteria. However this is based on this year criterion and next year it can change. And if the criteria remains same, then you can appear for Officers' Training Academy as the maximum age limit is 25 years. Read our article http://competition.careers360.com/articles/cds-eligibility-criteria