The Combined Defence Services (CDS) exam is conducted by the UPSC (Union Public Service Commission) to recruit officers for:

      A)    Indian Military Academy (IMA)

      B)    Indian Naval Academy (INA)

      C)    Air Force Academy (AFA)

      D)    Officers Training Academy (OTA – Short Service Commission)

     CDS Selection Process consists of 3 stages:

      1)     Written Exam

      2)     Staff Selection Board Interview (personality + psychology + group tests)

      3)     Medical Examination

Final selection is based on score obtained in first two stages. 
The Combined Defence Services exam is conducted twice every year by the UPSC, usually in April and September.

Check for notification at https://upsc.gov.in

Eligibility Criteria:

Age Limit 

IMA: 19–24 years, INA: 19–24 years, AFA: 20–24 years, OTA: 19–25 years ((approx. range, exact age varies in notification.)
(Exact cut-off dates change every notification, so it is advised to check the official UPSC notice.
Minimum age is 19 years. The upper age depends on the academy of choice)

Educational Qualification:

IMA / OTA → Graduate in any discipline

INA → Engineering degree

AFA → Graduation with Physics & Mathematics at Higher Secondary level (10+2) or Graduation in Engineering.

(However, Final-year students can also apply, the candidate must pass graduation exam before training starts)

Marital Status

IMA, INA, AFA → Only unmarried candidates

OTA → Both unmarried and (in some cases) married candidates eligible

Graduation-level knowledge is expected in exam.