as per the guidelines, the required eligibility for CDS Examination is that , all interested candidates will have to complete their graduation and only then can apply for the examination. The candidates applying will have to either have passed their final year examination or bet yet to pass their final year examination.
You can attend CDS exam but in your final year provided that you dont have any backlog in your past years.You cannot apply if you have any visible tattoo on your body.So you need to be between 19-24 years of age in order to apply and should also be unmarried.Finally graduate students or those appearing in final year can only apply.
Hope it helps!!