When you finish this article, you'll be older than when you
started it! That's because growing older is a gradual, life-long
process. Aging starts at the moment you are born.

How old is old? You may think you're young, but someone
your age is old to a baby brother or sister. The longest human
life span -- the length of time between birth and death -- isn't
more than 120 years, and hasn't changed much over the
course of history. But life expectancy -- the average length of
time a person of a particular age may expect to live -- has
changed dramatically.

If you were born in 1900 in this country, you could have
expected to live 47 years. A baby born today can expect to
live almost 76 years. Better nutrition and clean drinking water
as well as biomedical discoveries such as the immunizations
you get to prevent measles and other diseases and medicines
that treat strep throat have all brought about this change.

We all age differently. Some people get deep wrinkles, others
don't. You can get clues about how you will age by studying
your parents and grandparents. Scientists believe that
heredity, or the passing of traits from parents to their children,
is responsible for about 30% of the differences in life


Next time you're at a family gathering, notice if there are any
striking patterns. Do you have many tall relatives? What color
is their hair? The way you look is determined by genes, which
hold the instructions our bodies use to make our features. For
every feature (such as eye color, dimples or face shape), a
baby inherits two sets of genes, one from its mother and one
from its father. Which genes the baby gets from each parent
is random, like flipping a coin. There are thousands of genes,
thousands of features, and millions of possible combinations --
which is why you may have different hair color than your
brother or sister.

Genes also hold information that causes diseases. These
diseases are called genetic, or inherited, diseases. Cystic
fibrosis is an example of a genetic disease. Some genetic
diseases don't show up until later in life, even though the
genes are present from birth.

Genes may play a part in the changes in your body when you
become a teenager, as well as the normal changes that
happen as people grow older. As people get older, they may
have trouble with eyesight and hearing and their senses of
taste and smell may not be as sharp. Appearance changes
too; hair turns gray, and skin gets wrinkled. Older people may
slow down physically, and their memories may not be quite as
sharp. But people over the age of 70 still have hobbies and
interests, play sports, make jokes.