Because time slows down for an object in motion, since everything's in motion (the Earth, the solar system, the galaxy, etc.) we're basicly whizzing through time-space even when idling on planet Earth.
Neat article here about all these velocities including the speed we're all moving all the time as the galaxy rotates. (483,000 MPH by the by.)
"And how fast is the Milky Way Galaxy moving? The speed turns out to be an astounding 1.3 million miles per hour (2.1 million km/hr)!"
Consequently, there is no rest state where you're never moving. As a result, we're all a bit older than we think due to relativistic time dilation. Difference is perhaps just a second or so, but it's not nothing.
If any math nerds wanna do the math it's beyond me but I'd love to see the numbers.