My doctor told me you have androgentic alopica, but my father was in problem at age 40, my elder brother age 30 has never problem of hair loss.?

This prove nothing: The degree to which men get androgen-dependent hair loss varies tremendously within families. You're free to believe or disbelieve as you wish; time will show whether your physician is correct. If so, you can get hair-restoring medicines, get hair transplants, or get physically fit, shave your head, grow some facial hair and look like a stud.