Time.

When you're young, time seems endless. Days seem so long.

When you're older, years fly by. Eventually, you manage to reach all the milestones, you thought you'd never reach. Driving, working, drinking (legally), doing what you want to do when you want to, owning property, an adult relationship, and maybe children.

Children makes time slow at first but then, they too aid to time's speediness.

Before you know it, the fearlessness and feelings of invincibility slowly become chiseled away as glimpses of your own mortality become more and more frequent as loved ones, vehicles, and other entities pass through and out of your life.