Stress or Hereditary?

My grandmother's hair was all gray by 30 years old. Meanwhile, my dad is 53 with jet-black hair with a couple gray streaks. Why do some people gray pre- maturely while others die with their original color? Is it due to a change in pigmentation or simply due to hereditary? Is stress a dominant factor when determining at what age a person will begin to gray? I feel that stress could be a relevant factor in this mystery. Just as stress will make a person appear older at a younger age, shouldn't it also be able to gray a person’s hair at a quicker rate? So just a thought to keep in mind... RELAX and your youth will be prolonged.