Steve - First of all, some cells in our body don’t divide such as nerve cells and so they really aren’t. They last our whole lives but even the cells that do turn over- a new cell is generate from the division of an existing cell. The age of an existing cell basically gets transferred. The new cell remembers how old the other cell was. One of the important things that is transferred are the chromosomes. The ends of the chromosomes called telomeres get shorter every time a cell divides. They are a very useful counting mechanism. These telomeres shortening is one of the important counting mechanisms that tells our cells how many times they’ve divided and how old they are.

Ben - This is basically because hair is dead. Once it comes out of you, once it’s grown, all the melanin it will ever have has already been put into it. It gets broken down by the sun but in your skin the melanin gets replaced when melanocytes put new melanin into your skin.