This is a hazard we don't face in UK fell races: a 16-year-old competitor in the junior Bird Ridge race in Alaska was killed by a bear, while returning to the start after finishing the race. http://www.ktva.com/apd-bear-mauling-bird-creek-362/
Bird Ridge is a 3-mile uphill-only race with 3400 feet of climbing; the junior race goes half-way, although juniors are actually allowed to run the full course. There appears to be no lower age limit: the 2016 results show a 4-year-old completing the junior course, and an 11-year-old on the full course.