Dear Ashok,
For all purposes age is calculated and expressed on the basis of completed years only. So is the case with any employment whether in Private or Public for entry into and retirement from service as well. But the difference with respect to Central and State Government Services, in respect of retirement is that in stead of the actual date of attainment of retirement on superannuation, Govt employees are allowed to retire on the last working day of the month irrespective of the actual day on which they complete the age of retirement. For example, if the age of retirement is 60 years, a govt servant born on 2-06-1958 and joins the service the service in 1983 i.e., at the age of 25, he would be completing the age of 60 on 01-06-2018 but allowed to retire at the end of the month i.e on 30-06-2018 only. If he was born on 01-06-1958, he would have been retired on 31-05-2018 itself because that is the date of his completion of 60 years of age. So, it is the date of birth and not the date of joining which determines the date of retirement. For example, another person born on the same date i.e., 02-06-1958 joins the service at the age of 28 i.e in 1986, he would also be retiring on the same 31-05-2018. Better you go through the Govt pension rules. Earlier, Govt. employees were retired on the exact date of completion of the age fixed for retirement. But, later, for the sake of administrative convenience it was so amended.