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 lines for boys free of hormonal woes.

At the venerable St. Thomas Boys Choir, where Bach once drilled pupils in their scales, leaders have redoubled recruitment efforts and taken in boys at a younger age to make sure the choir has a full stock of voices ranging from the deepest bass to the most clarion-pure soprano. Children whose voices are deepening wait out the change by working the ticket booth.

The cause of the shift remains unclear. But some choir leaders say it is having a subtle effect on their music, and it’s not just that they have to buy more acne medication. The younger the boy, the less life experience and maturity underpins the complex emotions in what they sing, even if they’re more willing to study their scores instead of pining about romance.

“We have only a short time, from age 9 until 12, to squeeze in all the musical training for the boys,” said Stefan Altner, manager of the St. Thomas Boys Choir and once one of its singers. When he started working at the choir in 1993, most voices broke when boys were 14 or 15, he said. Now the average is closer to 13.

Because most boys join the choir when they are 9, even small changes are felt in the balance of the singers, he said, because after their voices deepen, the boys sing lower lines or drop out of the choir completely. In 2008, the choir added a nursery school, and in 2010, it opened a primary school as a way to identify and attract talent at a younger age.

“We want to find a way to have them from kindergarten all the way until Abitur,” the German high school graduation, Altner said.

The changes introduce an artistic conundrum to the hotly disputed studies of puberty, which some data indicate is starting earlier in girls, at least in the United States. Less attention has been focused on boys. Hypotheses about possible causes of earlier puberty in girls include improved nutrition, increased obesity, exposure to chemicals that mimic estrogen and changes in social pressures.

Among girls, researchers note that although studies have found that breast development may be starting earlier, the onset of the first period has remained constant, lengthening puberty rather than accelerating the process. Among boys, some studies have suggested that puberty might be starting earlier, but scientists agree that more research is necessary before they can say for sure.

“That puberty might start earlier in boys nowadays than it did decades ago is likely,” said Jean-Pierre Bourguignon, a professor of pediatric science at the University of Liege who has done research on early-onset puberty. But he cautioned that a change in the timing of when boys’ voices break could, in theory, be separate from a shift in puberty as a whole.

Maturity aids interpretation

The St. Thomas Boys Choir’s long history has made it ripe for special scrutiny, and studies of records from Bach’s time — he led the choir from 1723 until his death in 1750 and is entombed in St. Thomas Church — suggest that during his tenure, most boys’ voices started to change between ages 17 and 18. The age spiked over the eight-year War of the Austrian Succession, which began in 1740 and plunged the city into poverty, leading some scholars to wonder whether diet and health affected the boys’ voices.

“In terms of the musical ability, the older the boys are in the choir, the more they can relate to the music in a way like grown-ups,” said Mogens Halken, the head of music at Copenhagen Municipal Choir School, which has collaborated with researchers interested in the accelerated timing of voice change in boys. “If we take them in earlier, of course they will be more childish, and maybe they cannot relate to the music in the same way.”

The Danish school keeps careful records about when its boys’ voices crack, which is what attracted the researchers. Over the past 30 years, the timing has shifted six months earlier on average, Halken said. The school plans to move boys into the choir half a year sooner to make up the difference, he said.

A storied institution

In Leipzig, an east German city of 530,000 where opera singers in stage makeup take lunch breaks at a chain restaurant on the city’s largest square, the St. Thomas boys are an institution. The 97 choir members, who range in age from 9 to 19, live in a boarding school on Sebastian-Bach Street.

There, the older boys — who sing the lower tenor, baritone and bass lines — look after the younger ones. All receive a rigorous musical education. Boys whose voices are changing, a process that takes between three months and a year, take a break from the choir to study music theory, do light voice training and sell tickets and CDs at concerts. The group performs 100 times a year and regularly tours around the world.

The tours are “a bit like a class trip, and then concerts, that’s kind of like a football game,” said Oskar Didt, 16, a baritone who joined the choir when he was 9 and “they were a bit short on sopranos.”

In Leipzig, the issue has inspired local research and efforts to help prolong the boys’ soprano careers by keeping them singing the high parts until their voices change.

“Every choirmaster tries to cast the older boy singers as close as possible to the onset of puberty and the voice change,” said Michael Fuchs, who heads the voice, speech and hearing disorder department at Leipzig University Hospital and is a former member of the boys choir. Because research on testosterone levels has advanced in recent years, “we are able to predict with relative precision when the voice change will start,” Fuchs said.

But some aspects of the choir remain unchanged over the centuries, said Altner, the choir manager. Bach’s melodies are the same, as is their power.

“For the boys,” he said, “it’s very impressive to see what they have done to the audience.”

Special correspondent Petra Krischok in Berlin contributed to this report.