Grey hair, dandruff. Really serious?

I started getting grey hair when I was in 9th Standard (14 yrs). I did undergo treatment for 3 years at the most reputed and trusted skin centers at Delhi but in vain. I gave up on all the fast food, chocolates and other similar food items as suggested by the doctors. Without giving any reason to why my hair is turning grey. I am now 24 and I have lots of grey hair. Now I take no medicines and eat whatever I wish to but I really wish to know the cause for this problem. I have never coloured my hair except in past two years I coloured my hair twice (loreal hair colour). I wish to know Is this some kind of disease that my black hair is slowly turning grey over years? or is it a symptom for some other problem like stress? Is it genetic ? but so far none in my family has faced this. Or is there completely something else which I am just neglecting and I am undergoing something really serious? hair fall, dandruff is a continuous affair and lot of hair thinning thus hampering growth.