The first Adventure a lad or lass takes, which often tests their capacity for serial immortality. Because so few individuals are blessed with this gift, but most first-time Adventurers are reckless, many of them do not survive their first Adventure. The high mortality rate has led this to be considered a fool's errand.
Many first-time Adventurers take on the first task set before them -- usually a local or regional legend of long-standing provenance. Of course, any beast or legend which has a long-standing history also tends to have a propensity for lethality, or little reward for the danger involved. Otherwise, some other Adventurer--probably someone of known mortality--would have already overcome to the task.