The best part? He was twelve years old at the time.
Here's the account from Edward Hitchcock, geologist and the third president of Amherst College:
"About the year 1802, (possibly a year earlier or later,) Mr. Pliny Moody of South Hadley, in Massachusetts, then a boy, turned up with a plough upon his father's farm in that place, a stone, containing in relief five tracks of the Ornithoidichnites fulicoides, and it was put down as a door-step, because it contained tracks, and the neighbors used facetiously to remark to Mr. Moody, that he must have heavy poultry that could make such tracks on stone.
"After Mr. Moody (junior) had left home for school or college, Dr. Elihu Dwight of South Hadley purchased this stone, because it contained these tracks. It was retained by him nearly thirty years, when I purchased it for my cabinet, I think in 1839. Dr. Dwight used pleasantly to remark to his visitors, that these were probably the tracks of Noah's raven."
There's just so much in this passage that I love: the fact that at age 12 he's plowing the field; the fact that they used the slab as a door-step; the fact that Dwight referred to it as "Noah's Raven;" and the fact that the guys' names were "Elihu" and "Pliny."
But I think the best thing is that you can just picture the poor kid cringing every time one of those neighbors facetiously remarked on "heavy poultry..." :-).
In case you're wondering, Pliny seems to have attended Middlebury College and Williams College, married in 1819, had two children, and died in 1868. The tracks are, apparently, still in the collections of Amherst College (the alma mater of his son, Plinius).
Pliny, though, isn't the only youth to have discovered significant fossils: The Dallas Museum of Natural History has in its collections a baby nodosaur (possibly Pawpawsaurus) found by then twelve-year-old Johnny Maurice, while the Fort Worth Museum of Science and History has a Tenontosaurus found by then five-year-old Thad Williams. The original Bambiraptor was found in 1995 by then fourteen-year-old Wes Linster.
Also, Dakota, an Edmontosaurus, and one of the most complete "mummy" dinosaurs currently known, was found in 1999 by then high school student (15 or 16 years old) Tyler Lyson. Lyson is now director of the Marmarth Research Foundation and is pursuing his Ph.D in paleontology at Yale.
Sources (in addition to those linked inline):
Edward Hitchcock, Report of Ichnolithology or Fossil Footprints, in Silliman et al., American Journal of Science and Arts 47(2): 292, 297 (1844).
Edward Hitchcock, Ichnology of New England: A Report on the Sandstone of Connecticut Valley, especially its Fossil Footprints (1858).
Edgar J. Wiley, Catalog of the Officers and Students of Middlebury College 1800-1915 (Middlebury College 1917).
W.L. Montague (ed.), Biographical Record of the Alumni of Amherst College 1821-1871 (Williams Press 1883).
Louis Jacobs, Lone Star Dinosaurs, Texas A&M Press (1995).
Phillip Manning, Grave Secrets of Dinosaurs: Soft Tissues and Hard Science, National Geographic (2008).
Picture is from Ichnology of New England, showing the "Moody Quarry."