The Fleabirds are a group of people indigenous to North America who have rejected the collectivism and sedentary lifestyle inherent in the Cosmic Era. Rather than living in the gleaming arcologies, steaming geofronts, or the great heaps of the urban rims and favelas. Rather, they live in caravans that travel the countryside of the continent.
A race of beings actually IS invunerable while in adolecence. They age, but cannot be killed. A miniority of the race (1 in 20) does not become invunerable, but rather becomes immortal at some point in thier adolecence, and ceases aging while being vunerable to death in all the normal ways. Another minority (also 1 in 20) Permanatly becomes invunerable after adolecence, but ages twice as fast as the race normally does. There is no known way to test for which of the three traits an individual manifests and the three traits cannot mix, as they are tied to the same gene, as it were.