Of all the times that mandatory minimum sentencing laws forced Kevin Sharp, a retired federal judge who presided in the Middle District of Tennessee, to mete out a punishment that he felt was too harsh, he thinks often of Chris Young, whom Sharp sentenced to life in prison.

Young was twice convicted, at ages 18 and 19, in state court on drug charges, the second time for possessing an amount of cocaine weighing about as much as a quarter—as in a 25-cent piece—Sharp said in an interview.