Dear Seema,
Basically, employment is also a contract falling well within the Indian Contract Act,1872.
Therefore, the contract of employment between the employer and employee comes to an end with the employee attaining the predetermined age of superannuation mentioned in the contract or in the service regulations applicable to the employee.
In such a situation, the extension of service given to a particular employee on the exigencies of work by the employer after his attaining the age of superannuation is purely a matter of discretion of the employer and as such the same cannot be cited as a precedent by the employees retiring subsequently.
Anyway, to avoid such unnecessary claims, you first permit the particular employee to retire on the specified date and settle all his terminal benefits. After a gap of one month or so reappoint him as a fresh employee on his written request as a fixed term contract employee. Alternatively, You can also engage his services as a consultant on retainer basis but his present cadre of employment will not justify it.