What causes human bodies to age? Here's what scientists know about the biology behind growing old.

Everything grows old. As you age, you may notice an extra wrinkle on your forehead or lower energy levels.

To find out what's going on inside our bodies that causes these age-related changes and declines, he journal Cell assembled a group of researchers in 2013. That team, comprised of scientists studying different aspects of aging, reviewed all the existing literature on aging and wrote anl overview.

"The Hallmarks of Aging," as their paper was called, summed up everything that happens in our bodies biologically as we get old, and categorized those processes into nine"hallmarks."

They are:

Here's what each of these hallmarks means and how they work.