I would like to say that "curiosity" is a wonderful thing, so keep it up!
To answer your questions:
First, why do "young people" have hair of different colors? If you
can answer that one, then you can make a good hypothesis about why
older people have white hair.

Your 2nd question is a science history question, and a good one!!!
The first time anyone actually "saw" a cell was in 1665, when Robert
Hooke looked at cork sections. He called the little spaces "cells."
The microscope was invented in 1609 and it took a while for anyone to
really get around to seeing anything that small. In 1676, a guy named
Anton van Leewenhoek decided to look at some pond scum and he
described the first living, single celled animals. As far as human
cells go, van Leeuwenhoeck, Marcello Malpighi and Jan Swammerdam
described human blood corpuscles in the very late 17th century. The
first picture of an intact cell, taken with a camera connected to an
electron microscope, was done by a scientist named Keith Porter,
along with Albert Claude and Ernest Fullam in 1945. in the year 2000,
we have technologies that allow us to look inside of living cells and
record digital video images.
________________________________________________________________
For all of the questions: students interested in "cells and how they
are studied" as well as "what is a cell biologist?" can contact the
American Society for Cell Biology (ASCB) for free brochures and
on-line information at:

http://www.ascb.org
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I was just talking about white hairs with my husband yesterday.He thinks
our hair is getting gray, so the hairs aren't actually white; they just
don't have as much color as before. I compared a "white" hair and a brown
hair from my head with a white hair and a black hair from my dog by looking
at them all in a microscope. I think my "white" hair is actually white -
it looks like it doesn't have any of the colored molecules that make my
brown hair brown. Maybe the colored molecules are melanin, like in skin
and sun tans; I'm not sure.

But my dog's hair looked different - the black hair was so dark that no
light could go through it, and the white hair had something dark in the
center. So now I'm curious about how dog hairs are different from human hairs.

Hairs must turn white when the hair follicles where they are made stop
making colored molecules to put in the hair. There are also genes that
tell whether you will get white hair when you are young or whether you will
get very old and still have a lot of dark hair. I got the dark hair genes;
my younger brother has the white hair genes, so he looks older than I do.

You can get a good microscope that's easy to use at some place like Radio
Shack for about $10. It looks sort of like a rectangular flashlight.

White or gray hair is actually hair without color. We have
specialized cell in our scalp that make hair. Then we have other
cells that make the color that goes in the hair. In older people,
these cells that make the color for hair stop working for some
reason. The cell that make the hair itself still work and grow hair,
but just without color. I am not sure why the color stops being
produced. Maybe someday you can figure it out and then explain it to
me!

For your second question, lets think about it. How big are cells? Why
didn't people just know that our bodies were made up of cells? Cells
are very very very tiny, you can't see most cells with just your
eye--they are too small. So you need a special magnifying glass to
allow you to see something so small. Another property of cells that
makes them even harder to see is that most cells are clear, so not
only are they tiny, but they are also basically invisible! It was not
until the late 1600's, when Antonie van Leeuwenhoek invented a simple
microscope. He could magnify things to make them appear 266 times
their actual size. This allowed him and others to view cells for the
first time, which eventually led to the discovery that our bodies are
composed of billions and billions of cells.