Michael Hayden is the youngest I know of. Completed his first at 16, an has done over 10 ultra's. Not sure if he is still the youngest, that was a few years ago.
official because if a 10 year old just goes and runs 100 miles thats not official and therefore we'd never know. I can't speculate that no one under 16 ever ran 100 miles just not in an event. and yeah I'm 18 done an ironman and gonna do my first ultra next month