As per the best of my knowledge, you can attempt css while being a part of any armed services. But the process of CSS from armed forces is little bit different and easy. If you are member of any armed forces, you have to attepmt only one written paper of General Knowledge type and after that psychological test and interview process is left. So if you are 26 years old it is a best opportunity for you to join navy, taste it and reap the benefits of navy to be the part of civil services.

I am not confirmed that the process of moving into civil services is same in navy too as I have mentioned above because the source through which I
have come to know about this information is of Pak-Army. But I would recommend you to consult this interpersonally with any armed forces personnel. And I think that person can lead you to the finale conclusion that either you are going for navy, or not.

Regards and Reverence

__________________
Assistant Security Officer- BS-16
Ministry Of Defence, GHQ.