I’m just thinking, because although the effect is minor, it would make one older than the other, right?
Yes. Together-in-Space-151954.shtml
(Don’t know how to do links on iPhone.)
One twin is always older than the other, even if none of them have been in space.
Castor and Pollux?
There’s a pair of twins who are both astronauts:
Here’s a link for the Kelly twins referenced above.
But as the URL says, they’ll be *together *in space. I assume the whole point of the OP is the relativistic time dilation effect that would be measurable if one twin stays on earth and the other is moving at a higher speed.
Of course, instruments these days are probably capable of measuring the ordinary differences just from jet flights and various altitudes and latitudes on earth, so any differences caused by space travel would be just a couple of decimal places over.
Darn technology. Takes all the wonder out of science. 
This appears to be the link that D18 was trying for. For future reference, if you paste in the complete URL, the board software will automatically turn it into a link.
IIRC, there is barely measurable time dilation every time you go up in an airplane due to being further out in the gravity well. They measured it by putting an atomic clock on a plane.
Forget Time Dilation effects! They should leave one Kelly on earth and see if they can communicate via telepathy!
We oughtta try out some of Heinlein’s ideas. It’s time for the stars!
There’s nothing magical about being “in space” that causes time dilation. You can do the same by just walking to the grocery store and back.
What about Twin Pigs In Space?
The paradox doesn’t require twins, either. My mother could go on a space trip of sufficient length and velocity so as to come back “younger” than me. Or my wife, who happens to be the same age as me as of now.
I know, it’s just that one usually doesn’t go thousands of miles per hour for weeks+ nonstop on the ground. Thanks for the answers everyone!
Does this mean that time passes faster in Denver than in Miami or slower? And over the course of a lifetime, assuming you never went anywhere else, what would this total difference work out to? (My guess is somewhere within 100 ns to 100 µs.)
Time in both Denver and Miami is slowed a bit relative to freefall, but it’s slowed more in Miami. So Denver’s faster.
So, compared to Chicago, say, you’ll live longer in Miami, all other things being equal? Because everything’s going faster in Denver, and so, you’ll die sooner, although, not by much. Right? This is so confusing to me for some reason.