A flying saucer crashed in the Mojave Desert and its inhabitants turned out to be alien slaves, bred to be super intelligent and strong, and controllable by their Overseers. These Tenctonese, or

The Alien ship that crashed is the same one in the V series.

The Aliens aged at the same rate we did until they turned 16 then the aging speed dropped to half. We learn about this when Susan goes to get a job and is turned down because she is just too old for the job.