Upper age limit for CSS CE is 28 (as of now), for in service government employees it is relax-able by 2 years (subject to their respective departmental approval and recommendation). Therefore if you are in service for continuous 2 years and you get a letter stating the same from your department and you are less then 30 years on Dec 31, 2011 then you are definitely eligible...hope this helps.

By the way it is also clarified in the following rules page
Rules For CSS Competitive Examination - CSS Forums