The question is pretty simple: what is happening molecularly when hair turns grey or white? I would imagine that it is due to the lack of a particular compound. I'm also interested in why some people's onset of grey hair happens much later than others.
Each individuals hair colour is determined by the particular pigment they produce (called melanin - the same stuff in your skin that makes you tan).
As the body ages this pigment (produced by the melanonocytes - cells that reside in the hair follicle) is produced less and less, until the hair is no longer coloured, and appears grey.
This is unique to each individual because it is a genetic (and therefore highly heritable) trait. Because it is in no way linked to mortality there is no selection pressure against greying hair.
The reason that the pigment is not longer produced is the gradual depletion of the stem-cell pool with age. This is common to many tissues, hair follicles being just one. As the 'pool' of stem cells is depleted, the melanocytes are no longer replaced as frequently, and thus less pigment is produced as we age.