I Am 21 Years Old Male. Some Of My Hair

I am 21 years old male. Some of my hair had turned white. I had completed some medicine course but the white hair does not turned black. Please suggest something to make my hair dark black permanently. I am very tensed about it.I am a student and i feel very embaraced when other students says that you are old. Is it possible that my white hair will turn black again doctor please help me.