Tenor Boy

It's practically musical theatre law. The frequently good-looking, almost always lovestruck, and without a doubt naive young man central to the story must sing with lyrical, boyish grace. In other words: he's a tenor. In a large portion of opera (particularly Verdi and Puccini), the hero is a tenor no matter what his age or personality.

Contrast Badass Baritone.

Examples of Tenor Boy include:

There are practically too many examples of this to name, so it might be easier to start by naming exceptions.

The particularly boyish Arpad in She Loves Me is a lyric baritone.
In Bock and Harnick's more famous show, Fiddler on the Roof, Perchik—though only arguably classified as an ingenue—is also a lyric baritone.
Jean-Michel of Jerry Herman's La Cage aux Folles wavers somewhere between baritone and tenor.
The same goes for Freddy in My Fair Lady.
Film example: In Disney's Aladdin, Aladdin is a baritone. Interestingly enough, Brad Kane, who provided the singing voice for Aladdin, also played Arpad in the 1992 revival of She Loves Me.
Rolf in The Sound of Music is arguable. While definitely a baritone, his status as an ingenue is very debatable.
Anthony in Sweeney Todd: The Demon Barber of Fleet Street has the range of a typical lyric baritone, though he can be played by a tenor.
Gilbert & Sullivan with Grosvenor and Strephon in Patience and Iolanthe, both being lyric baritones (though Strephon originally played by bass Richard Temple). Somewhat averted in Ruddigore: Dick Dauntless doesn't turn into an evil baronet and he has the romantic-style music, but he's also, well, a dick, and doesn't get the girl. Also lampshaded like hell in the Act II opener of Utopia Limited, with Captain Fitzbattleaxe's song about how you can't sing in those high ranges if you're actually overcome with emotion instead of just acting.
Porgy and Bess, an ingenue in love if not in age, is a bass-baritone; the bad guy Sportin' Life is a tenor.
Verdi's opera Rigoletto subverts the trope. The Duke is a good-looking tenor that all the girls fall for, but he's the opposite of an innocent - he's a Manipulative Bastard who doesn't care what happens to the women he seduces and abandons.
- Pinkerton in Madame Butterfly is a heartless bastard, very similar to the Duke.
- And in Janacek's Jenufa, we have two tenors: Steva is a jerk who knocks up poor heroine and then leaves her, Laca is a neurotic who cuts her face in a fit of jealousy (that's why Steva leaves her, What Measure Is a Non-Cute??).
Surprisingly averted in Billy Budd, in which the titular character is a baritone.
Inverted hard with Peter Grimes - the titular character is the furthest thing from an ingenue.
- Britten subverted this all the time. Living with a tenor who was definitely not an ingenue might have something to do with it.
  - Aschenbach from Death in Venice is an ageing writer with a fondness for Bishonen.
  - The Madwoman in Curlew River is a drag role.
  - Captain Vere from Billy Budd is a father figure in the flashback part and very old (or possibly even undead) in the Prologue/Epiloge.
  - Played straight, though, in Albert Herring, Owen Wingrave, and his adaptation of A Midsummer Night'sDream.
CB of Starlight Express, who while starting out like a stereotypicial Tenor Boy, is soon revealed to be a giddy and sociopathic Ax Crazy Serial Killer.
Dr. Horrible is a tenor-voiced Adorkable, naive, (mostly) innocent, hopelessly romantic Shrinking Violet... who is a Well-Intentioned Extremist Villain Protagonist.
In the opera Salome, Jokanaan is a baritone, while Herod the king is a tenor. Played straight with Narraboth, however.
While a side character, there is a tenor boy in The Cook the Thief His Wife And Her Lover who has ambigious age and has a beautiful voice.

Now for the straight examples (the more notable ones at least)

Raoul from Phantom of the Opera. Notable for being generally hated by most of the~~phans~~ fans.
Marius Pontmercy in Les Misérables is rather tricky to place. He is often played by tenors, but has a slightly lower tessitura. However, he does fit the stereotype perfectly.
Jonathan Harker in any and all Dracula musicals. Again, see the source material.
While we're on a roll, let's throw in the very similar Charles Darnay in the new A Tale of Two Cities musical.
Jack in Into the Woods, sooo much.
Buddy Foster in Sideshow.
Quasimodo in The Hunchback of Notre Dame stage version.
The titular character in Pippin.
Link in Hairspray.
Henrik of Sondheim's A Little Night Music, though a bit unconventional, fits this trope quite well.
Frankie Epps from Parade. Of course, most of the roles Jason Robert Brown writes are tenors, regardless of type.
Tony in West Side Story. He is, after all, based on Romeo and Juliet, who by stereotype is pretty much incapable of being anything but a tenor.
John Savage in the film version of Hair (theatre).
Bobby Strong in Urinetown.
Melchior Gabor in Spring Awakening.
While usually a bit older than the average ingenue, Leo Bloom in The Producers fits.
Mickey Johnstone and Eddie Lyons in Blood Brothers.
Gilbert & Sullivan have quite a few straight examples as well, with the most famous being Nanki-Poo and Frederic from The Mikado and The Pirates of Penzance, respectively.
Alfred in Tanz der Vampire. (Which makes it a bit of a soundtrack wallbanger moment for this troper if Herbert is sung as one too...)
Charlie in Brigadoon. Tommy, being older, is a baritone.
Hero in A Funny Thing Happened on the Way to the Forum. Played for laughs, too.
Another film example: Christian from Moulin Rouge has both the range and the standard personality.
Lensky from Eugene Onegin. Innocent, naive, and poet.
Rodolfo from La Boheme.
Des Grieux form Manon Lescaut.
Tamino, Belmonte, Ferrando and Ottavio - all those gorgeous-singing, plain boring Mozart tenors. The girls and the basses steal the show generally.
Albert Herring - another naive Britten hero - is a tenor.
The tenor roles in Donizetti's three opere buffe: Nemorino in L'elisir d'amore, Tonio in La fille du régiment and Ernesto in Don Pasquale.
Parsifal, Stolzing, and Siegfried - the more "boyish" Wagner tenors.
- There's a point in one of the operas where Siegfried has to impersonate another character, a baritone. It's notorious difficult to pull off on stage. Some recordings of the opera get around it by having Siegfried sing the part in his normal voice and then editing it into a baritone.
Rudolf in Elisabeth.
The title character in Leonard Bernstein's Candide.
Macheath in The Threepenny Opera is sometimes played like this ironically, and many performances have him singing the "Epitaph" in a sincere tenor, just to accentuate what a two-faced bastard he is.
Viceroy Bánk the eponymous hero of Erkel's famous opera. Although he's far from being boyish.
Discussed in Lenny Bruce's bit "All Tenors Sound the Same".