Question: why do some people's hair go darker as they go older as well as their eyes?

Leo Garcia answered on 24 Jun 2010:

Hair colour does change with age – and people’s hair can change in colour when they are younger, and, when they get old, eventually go gray. The hair colour that children are born with often darkens as they grow up. The scientific name for the graying of the hair is: achromotrichia. The age at which hair goes gray is down to your genes – and in fact some people are born with gray hair.

Two genes appear to be responsible for the turning the hair gray: Bcl2 and Bcl-w. These genes cause melanin to stop being made in the hair root – and so any new hairs grow without any pigmentation. So, any changes in hair colour before graying will be due to changes in the amount of the melanin pigment within the hair root.

In terms of eyes – again, this is a question of the amount of pigment being produced, although this time it is within the iris, which is the coloured part of the eye. The pigment is called ‘eumelanin’. Babies are often born with no pigmentation of the iris, and will develop deeper eye colour as they grow up. Eye colour can also change, apparently, due to hormonal factors, such as pregnancy or during puberty:

http://www.eyecarecontacts.com/eyecolor.html

Exactly why and how this happens is not yet fully understood.