Thanks Jackson

Jackson,
First off, welcome to my strange and twisted world.

Your question: I have always wondered why certain hair turns gray earlier in life than other hair.

I shall do my best to answer this. I have to admit I was very intrigued by this question, then began to laugh as I read on. First thought I had: because there is no sunshine concerning the second part of your question, however, I know people that always wear hats, and still gre(a)(whatever)y quite easily.

Real Answers:
Dr. Desmond Tobin, professor of cell biology from the University of Bradford in England, suggests that the hair follicle has a “melanogentic clock” which slows down or stops melanocyte activity, thus decreasing the pigment our hair receives. This occurs just before the hair is preparing to fall out or shed, so the roots always look pale.

Moreover, Dr. Tobin suggests that hair turns gray because of age and genetics, in that genes regulate the exhaustion of the pigmentary potential of each individual hair follicle. This occurs at different rates in different hair follicles. For some people it occurs rapidly, while in others it occurs slowly over several decades.

In a February 2005 Science article (Nishimura, et al.) Harvard scientists proposed that a failure of melanocyte stem cells (MSC) to maintain the production of melanocytes could cause the graying of hair. This failure of MSC maintenance may result in the breakdown of signals that produce hair color.

There are other factors that can change the pigmentation of hair, making it lighter or darker. Scientists have divided them by intrinsic (internal) and extrinsic (external) factors:

Intrinsic factors:
Genetic defects
Hormones
Body distribution
Age

Extrinsic factors:
Climate
Pollutants
Toxins
Chemical exposure

My summary: So, possibly the fact that the hair on our head is exposed to the extrinsic factors much more than the other hairs is a possible explanation for our cranial hair turning gray more quickly.

Some hairs such as eyebrows and eyelashes are not regulated by androgens, while androgens stimulate axillary, pubic and beard hairs to grow and some scalp hairs to undergo miniaturization. The major circulating androgen is testosterone which is enzymatically converted by 5 a -reductase to dihydrotestosterone in the hair follicle. Also, in another section of this article, new cranial hairs are called vellus, while the hairs that have obtained color are called terminal.

My summary: The major circulating androgen is testosterone, which would then make it easier for the body to keep certain hairs healthier longer than other hairs. Also, goosebumps cause hair to grow, possibly speeding the cycle of growth, and therefore causing the cycles to expire more quickly. I think I get more goosebumps on my head, legs and arms than anywhere else, therefore resulting in those to turn gray more quickly.

If this didn't help, sorry. I will attempt to look for more answers if you are not satisfied.

Thanks to www.aad.org and http://www.loc.gov/rr/scitech/mysteries/grayhair.html