Ofcourse you can give cds exam until the age of 25 years if you have completed your graduation.there is no best year for attempting any exam it depends on your preparation if you are confident that you are going to crack the exam go for it dont waste your time for the right time to come.

Cds exam is conducted by UPSC hence it is hard you need to prepare for it do hard work smartly update yourself with ongoing news to get good marks in current affairs section as this is the toughest part of exam which keep changes with time.