Think of Aging as "whatever happens to your bodies as time passes." This description encompasses all the aging the body goes through, instead of the indicators of Aging that happen later in life, like grey hair and lines and wrinkles.
Some aging is caused by your body. Growing kids and teens going right through puberty. Other Aging accumulates as time passes, like skin surface damage triggered by sun exposure. Because of this, Aging is a mixture of physical changes and the impact of how exactly we look after ourselves.
Everybody knows the obvious signs of aging: lines and wrinkles, grey hair, a slightly stooped posture, perhaps some "senior occasions" of forgetfulness. But why do those ideas happen?