Harry Potter - with a beard?

When Harry Potter next returns to cinema screens in 2005's "The Goblet of Fire," the bespectacled wizard will face a foe who must not be named. It's an enemy so formidable, so terrifying, that not even a magic wand can defeat it (though a razor may stave it off - at least momentarily).

We're not talking about Lord Voldermort, Harry's arch-nemesis. No, the screen version of Harry Potter will face an adversary that has vanquished many a child star: adolescence.

Although J.K. Rowling's hero continues to age with each installment of the book series, Harry Potter actor Daniel Radcliffe is rapidly outgrowing the character. The actor, who turns 15 in July, may be too old to fly Harry's Firebolt broomstick by the time the fifth book, "The Order of the Phoenix," starts filming. That led "Potter" producer David Heyman to recently muse that he will have to find a new, younger cast to send to Hogwarts school.

It's not the first time questions have been raised as to whether an actor is the right age for a role. Not even older actors are exempt from such criticism. Already, Page Six wags are speculating on who will one day inherit the role of James Bond now that Pierce Brosnan's temples are graying.

In practice, however, Hollywood is often loath to revoke the license of actors whose ages no longer match those of their characters. Studios take liberties with the age of casts all the time - especially if audiences associate a star with a particular role.

"It's not to say that we don't have actors who are capable of making you believe that they are either younger or older than they are by quite a lot," says David Thomson, author of "The New Biographical Dictionary of Film." "But I think if you're going to repeat it, that's when it gets difficult, [especially with] the films where we have a history with the characters and the actors."

Case in point, Arnold Schwarzenegger's return to the role of a lean, mean, killing machine in last year's "Terminator 3." In theory, the muscular titan should have resembled the android he first played in 1984, but he looked a tad creaky around the joints. Similarly, Anthony Hopkins recently failed to convince as a young Hannibal Lecter in "Red Dragon," a prequel to 1990's "Silence of the Lambs." And Sean Connery made a belated return to Her Majesty's Secret Service in 1983's "Never Say Never Again" - wearing a toupée.

But if anyone deserves a raised eyebrow, it's Roger Moore who was 56 in his final outing as Bond. It raises the question whether audiences will line up to see a retirement-age Harrison Ford in a fourth Indiana Jones movie, if it gets made.

"With characters like [Indiana Jones] they're so much larger than life," says Dan Shaner, a Hollywood casting director. "I think that's what people buy into, more so than the actor."

These days, making a movie is so expensive that studios are reluctant to cast younger, untried actors in big roles, so they lean toward recognizable marquee names - even if a star seems too mature.

Twentysomething actress Rachael Leigh Cook was cast as a teen in "Stateside," a tear-jerker now in cinemas, for that very reason, says Todd Thaler, the movie's casting agent. "It's more of a challenge to find actors of renown who are in their teen years than to find slightly older ones who, for some, [require us to] suspend our disbelief and accept them as much younger characters," says Mr. Thaler.

But for every risky casting move that pays off - think 23-year-old Alison Lohman's portrayal of a 14-year-old in "Matchstick Men" - there are instances when actors reach too far. In "Riding in Cars With Boys," Drew Barrymore, then 26, played the mother of a 28-year-old actor. This fall, Angelina Jolie may stretch audiences' credulity when she tackles the role of Colin Farrell's mom in "Alexander." Blame James Cagney. In "Yankee Doodle Dandy" he set a precedent by playing the son of an actress who was 11 years younger than he was.

But the worst offenders may be films that feature older actors playing high-schoolers, a tradition that goes back to "West Side Story," "Rebel Without a Cause," or Mickey Rooney's 16 movies as small-town teen, Andy Hardy. Mr. Shaner says it's easier to cast nonteens because "you don't have to contend with parents and teachers and labor laws." The secret is consistency: If a 20-year-old is cast in the lead, the other actors should be in their 20s as well.

It doesn't always work, says Ty Burr, film critic of The Boston Globe. "You never, never buy somebody playing 10 years older, 10 years younger than who they are, with rare exceptions, and those tend to be character actors rather than stars."