The primary factor is genetics. Each person ages according to the genetic code stored in his or her DNA. Sometimes this means that a person will reach the age of 80 with his or her youthful hair color, while another person may be shock-white by the age of 25. Most people fall into the middle range. Your chance of going gray increases 10-20% every decade after age 30.

1) "A change in hair color typically occurs naturally as people age, usually turning their hair from its natural color to gray and then to white. More than 40 percent of Americans have some gray hair by their fourteenth birthday, but white hairs can appear as early as childhood. The age at which graying begins seems to be almost entirely based on genetics. Sometimes people are born with gray hair because it is passed down genetically. Some people use hair dye to disguise the amount of gray in their hair. The change in hair color is caused that occurs when melanin ceases to be produced in the hair root and new hairs grow in without pigment. Two genes appear to be responsible for the process of graying, Bcl2 and Mitf. The stem cells at the base of hair follicles are responsible for producing melanocytes, the cells that produce and store pigment in hair and skin. The death of the melanocyte stem cells causes the onset of graying." Source and further information: http://en.wikipedia.org/wiki/Gray_hair#Effects_of_aging_on_hair_color Further information: http://useinfo-grayhair.blogspot.com/ 2) "Table 1--Numbers of smokers and non-smokers with natural, grey, or balding hair according to age and sex --------------------------------------------------------------------------------------------------------- Men Women Hair colour ------------------------ ------------------------- or loss </=40 41-50 51-60 >60 </=40 41-50 51-60 >60 Total (No (%)) --------------------------------------------------------------------------------------------------------- Smokers Natural 8 4 0 0 10 13 1 0 36 (12) Grey 3 11 11 12 13 54 22 37 163 (54) Bald 2 30 22 49 0 1 0 1 105 (35) Subtotal 13 45 33 61 23 68 23 38 304 (100) Non-smokers Natural 2 13 3 0 30 21 23 4 96 (32) Grey 0 7 14 12 0 27 47 36 143 (47) Bald 6 12 9 34 2 0 0 0 63 (21) Subtotal 8 32 26 46 32 48 70 40 302 (100) Total 21 77 59 107 55 116 93 78 606" Source and further information: "Premature grey hair and hair loss among smokers" http://www.bmj.com/cgi/content/full/313/7072/1616 3) "Genes play a large role in graying, and there's wide variation among ethnic groups. Men tend to turn gray earlier than women. Smoking is linked to premature graying. In rare cases, gray hair may be a sign of illness. Among other reasons of hair graying are excessive intake of tea, coffee and alcohol, of fried and oily foods, spices and sour and acidic foods." Source and further information: http://english.pravda.ru/science/health/20-06-2006/82265-gray_hair-0 4) "People can get gray hair at any age. Some people go gray at a young age - as early as when they are in high school or college - whereas others may be in their 30s or 40s before they see that first gray hair. How early we get gray hair is determined by our genes. This means that most of us will start having gray hairs around the same age that our parents or grandparents first did. Gray hair is more noticeable in people with darker hair because it stands out, but people with naturally lighter hair are just as likely to go gray. From the time a person notices a few gray hairs, it may take more than 10 years for all of that person’s hair to turn gray." Source and further information: http://www.north-of-africa.com/article.php3?id_article=177 5) "Your chance of going gray increases 10-20% every decade after 30 years." Source and further information: http://www.disabled-world.com/artman/publish/gray-hair.shtml