Luciano Pavarotti’s death last year prompted much speculation about who will eventually replace the triumvirate of Pavarotti, Plácido Domingo, and José Carreras. Most of the discussion has focused on stars in their 30s, such as Juan Diego Flórez and Rolando Villazón. But what about even younger tenors whose careers are developing?

In New York, at least, the demands for talented opera singers will be greater than ever in the coming years. In 2009, Gérard Mortier will arrive to take the helm at New York City Opera, bringing ambitious plans for modern productions. The year 2013 will bring three major anniversaries — the bicentennials of Verdi and Wagner, and the centennial of Benjamin Britten. Who will be the rising stars in 2013? In particular, what tenors will emerge to sing roles such as Siegfried (in the Wagner opera of the same name), Alfredo (in “La Traviata”), and Manrico (in “Il Trovatore”)? Where are they now, and how are they preparing for their future roles?

Interviews with three young tenors — Americans Alek Shrader and Stephen Costello, and New Zealander Simon O’Neill — as well as several teachers offered a look at the next generation of tenors.

While prepubescent violin or piano prodigies appear fairly frequently, male opera singers have to wait on physical development. “Serious study should start anywhere from 21 to 23, and some can start as late as 28 to 30,” a teacher on the faculty of the Academy of Vocal Arts, Bill Schuman, said.

Lyric and leggiero (light and agile) tenors tend to bloom first. Mr. Shrader, 26, is a leggiero tenor who has already had considerable success. Having won the Metropolitan Opera National Council Auditions in 2007 and graduated from the Juilliard Opera Center this May, he will be an Adler Fellow at the San Francisco Opera for 2008-09. In the coming years, he is scheduled to sing several Rossini and Mozart roles, including Conte Almaviva in Rossini’s “The Barber of Seville,” at Opera Cleveland, Opera Theatre of St. Louis, the Bavarian State Opera, and the Théâtre du Capitole in Toulouse, France.

Europe is a popular destination for young singers, Mr. Shrader said, because it has many more opera houses than America does, and because the houses are smaller, so young voices have less space to fill.

Mr. Costello, also 26, is a lyric tenor, shading into a spinto — a somewhat heavier, pushed voice, in between lyric and dramatic (the heaviest and most powerful voice). A graduate of the Academy of Vocal Arts in Philadelphia, Mr. Costello has performed the role of Edgardo in Donizetti’s “Lucia di Lammermoor” at the Fort Worth Opera and the Metropolitan Opera. This season, he is engaged to sing Alfredo in “La Traviata” and the title role in Donizetti’s “Roberto Devereux” at the Florida Grand Opera, among other roles.

Mr. Costello’s trajectory reflects the fact that tenors with bigger voices may develop more unevenly. When Mr. Costello auditioned for AVA, he had not studied opera before, though he had played the trumpet. Because his voice was so unstudied, three of the five experts on the audition panel recommended rejecting him, said Mr. Schumann, who now brings up this anecdote in faculty meetings, to illustrate how a big tenor voice can develop from something not-yet-baked into something spectacular.

“It wasn’t obvious except to people who really know tenor voices,” Mr. Schumann said. “None of his vowels were correct, his use of resonance, [but] I liked the sheer instincts I saw in him: He had guts, and he’d throw his head back and go for it, just scream.” By Mr. Costello’s second year at AVA, Mr. Schumann added, “It became obvious that something was emerging.”

Dramatic tenors and heldentenors (the German term for tenors who sing Wagner roles) take the longest to come into their own. Mr. O’Neill, 36, started singing at university in New Zealand, but as a baritone. In his early 20s, he said, his voice started changing — sounding more tenor-y, with more squillo, or “ping.” But it took “10 years of hard work — and [of] making some atrocious sounds” to make a full transition to heldentenor repertoire. Having done so, “[I]t’s made my career take off,” Mr. O’Neill said. He has sung the role of Siegmund in “Die Walküre” at the Met and elsewhere, as well as the title role in Wagner’s “Parsifal.”

The difference between a leggiero tenor and a dramatic tenor is in large part physical. “The guys who sing Verdi and Wagner tend to look like football players, whereas the guys who sing Rossini and Mozart tend to look like swimmers,” a teacher at the Juilliard Opera Center, Brian Zeger, said.

But while it’s rare for someone to jump completely from one category to another, men’s voices do change up through their 30s, and singers, as they get stronger, can take on more and more demanding repertoire. “You don’t really know where your voice is going to end up,” Mr. Costello said. “The goal is to keep it healthy enough and strong enough.” For now, he is singing repertoire “on the lighter side,” he said: early Puccini, and Bellini and Donizetti, which he described as “like medicine for the voice.”

Deciding to become a tenor is itself a scary thing, a teacher at the Manhattan School of Music, Dona Vaughn, said. “It’s probably the most frightening thing that a young man will do in his life, other than getting married and signing a mortgage,” Ms. Vaughn said. “The tenor voice is a very naked instrument,” she continued. “It’s the most exposed voice we have in the business.”