Life-long Riddle

It can fly.
It can crawl.
But it cannot walk.

Sometimes you can buy it.
Sometimes you make it.
But you can't stop it.

You can be upon it.
But that might be considered cliche.
You can wish for it.
But anything can wished for.

It's old.
It's new.
It can change.
It can stay the same.

No one knows were it came from,
But then again,
No one's ever cared.
We have lived,
We are living,
We will live,
With it.

The answer to all of the above is
Time.

But do we really know what it is?
Yes,
sixty seconds make a minute,
sixty minutes make an hour,
twenty-four hours make a day,
seven days make a week,
fifty-two weeks make a year,
ten years make a decade,
ten decades make a century,
and ten centuries make a millennium.

But what is time?

Can you figure out the life-long riddle?