American men haven’t won an Olympic weightlifting medal since the 1984 Games, when the sport’s longtime behemoth, the Soviet Union, stayed home.

But a 15-year-old boy in small-town South Carolina is raising hopes for an American weightlifting renaissance. His name is C.J. Cummings, and at the 2015 USA Weightlifting National Championships this month in Dallas, he set a national record. In a sport where athletes typically peak in their mid to late 20s, this teenager lifted more than any American grown-up in his weight class...