"Six hundred years ago, the commanders at Snowgate and the Nightfort even went to war against each other. The simpering Lord Commander asked them politely to lay aside their quarrel, and incredibly, they did...because they joined forces to murder him. The Stark in Winterfell had to intervene, and take both their heads to save the Night's Watch."
Samwell Tarly, knowing that a Choosing will be occurring soon, decides to read about the history of past Lord Commanders. He is amazed that Osric was chosen at such a young age.
In the books
In the A Song of Ice and Fire novels, Osric Stark was elected Lord Commander of the Night's Watch at age ten, four hundred years before the War of Conquest, though he ultimately served as Lord Commander for sixty years.
Samwell Tarly specifically brings up the example of Osric Stark to point out that in the centuries-long history of the Night's Watch, boys younger than Jon Snow (who is around 16 years old at that point) have been elected as Lord Commander of the Night's Watch.