Hello!

The eligibility criteria for CDS exam should meet the citizenship criteria and then the age and academics criteria. For different courses in CDS there is different age criteria. The average criteria for all is 19-20 yrs min to 24-25 yrs max. Your age in December 2020 will be 23 yrs. So, you're eligible. For detailed eligibility check the link below: https://www.google.com/amp/s/competition.careers360.com/articles/cds-eligibility-criteria/amp

I hope this helps