Growing old is part of being a living thing. Every plant and animal must go through a cycle of life that involves a beginning, a middle, and an end. Actually, as soon as we are born we begin aging or growing older. But when we talk of growing old we think of the physical changes that occur when bodies cannot grow and repair themselves as they once did.
At about age 30 the signs of aging start to appear, though for most people the physical changes are not really obvious until many years later.