Dear Mr.Narayan paudel please refer to the below mentioned text for your info.
Deduction in respect of medical insurance premium [Section 80D]
(i) The following payments shall be deductible to the extent of the actual amount paid by any mode other than cash or Rs.15,000, whichever is less :
(a)Where the assessee is an individual, any sum paid to effect or to keep in force an insurance on the health of the assessee or on the health of the wife or husband, parents (whether dependant or not) or dependant children of the assessee;
(b)Where the assessee is a Hindu undivided family, any sum paid to effect or to keep in force an insurance on the health of any member of the family.
(ii) Such insurance should be in accordance with a scheme framed in this behalf by the General Insurance Corporation of India and approved by the Central Government in this behalf or in accordance with a scheme framed by any other insurer and approved by the Insurance Regulatory and Development Authority.
(iii)Where the assessee or his wife or her husband, or dependant parents or any member of the family is a senior citizen and the medical insurance premium is paid to effect or keep in force an insurance in relation to him or her, the maximum permissible deduction is Rs.20,000.
Now as per your query the answers are;
a) For Mr. B both the amounts are allowable deductions;
b) If Mr. A has attained the age of 65 yrs in the Financial Year then only he can claim the deduction for Rs. 20,000 unless only Rs. 15,000 will be allowed as deduction.