LEIPZIG, Germany — For 800 years, the St. Thomas Boys Choir has been filling churches with pure, young voices. Now it’s confronting a confounding phenomenon: Every year, those voices are cracking with teenage angst just a little earlier than before. Other boys choirs have been noticing it, too, as an unrelenting march of puberty sweeps voices into rebellion. Over recent decades, the already-short careers of their sopranos have started to end between six months and a year earlier, challenging them at times such as Easter, for which choral music such as J.S. Bach’s St. Matthew Passion was written with difficult...