>As in natural selection decided at one point to introduce death. It really is the case that older lifeforms don't die. All mammals do though.

Not weird at all. You sort of need this if evolution is to do its work... otherwise offspring must out-compete its parents and grandparents, who long ago colonized all of the good niches and left nothing for anything else. They'll die eventually, of course, but they'll almost certainly take the entire species with it because they were unadapted for changing conditions. Death is flexibility on an evolutionary timescale, and immortality is rigidity.