Robert Lesser, a spry 69-year-old character actor who appeared in "Die Hard" and had a recurring role in the TV series "Empty Nest," was surprised when, on a set of a legal thriller shot in Japan, the cast and crew brought out a birthday cake and started singing. It was May. Mr. Lesser was born in October.

Robert Lesser

"If I had it to do all over again, I would not have told them—in my awkward way—how grateful I was, but that, in fact, it was not my birthday," recalls Mr. Lesser. As it turns out, he says, "they got the bum dope from IMDb," a website that had listed Mr. Lesser's date of birth as May 28, 1938, tacking on four years to the correct date of Oct. 22, 1942. To add insult to injury, his birthplace was listed as Los Angeles.

"That really got me more than the age thing," says Mr. Lesser, who hails from New York City's Upper West Side. "I am from a real place, you know?"

Over the past 15 years, the Internet Movie Database, or IMDb, has become an increasingly popular source for biographical data, with profiles of more than four million Hollywood types. But many aren't exactly thrilled about its accuracy—or its revelatory powers.

For an industry where youth is a bankable asset—and where actors have lowballed their ages as a matter of tradition—IMDb has become something of a spoiler. In addition to listing actors' purported birth dates, the site offers free information about their given names, height, screen credits and even "trademark" features, such as Demi Moore's "husky voice." Some entries give measurements. For a monthly fee of $15.95, people can register to access more details, such as an actor's contact information.

Actress Amy Weber says IMDb has her age wrong—but won't say what it is. Globe Photos/Zuma Press

"Why does everyone need to know my bra size?" asks Amy Weber, whose public profile says that she is a 34C. Ms. Weber, who has appeared in TV series such as "CSI," also claims that her date of birth, listed as July 2, 1970, is incorrect, but declines to say what it really is. "I'm not in my forties," she says.

Actress Rosanna Arquette, 52, won't even check the site anymore because she finds some of the paparazzi photographs that keep popping up alongside her profile to be unflattering. "It's all about hurting people and making them look as bad as possible," says Ms. Arquette, whose IMDb profile lists the names of her three ex-husbands, and the dates she was married and divorced.

Such data points "can be a bit of an unfortunate reminder" for celebrities, says IMDb Chief Executive Col Needham, a Manchester, England-born techie (date of birth: Jan. 26, 1967, according to his IMDb page). "But people come to us because we are accurate and comprehensive." Due to its sheer size, Mr. Needham acknowledges there are bound to be mistakes, many of which he says are corrected within 24 hours.

The inner workings of IMDb—including how it compiles information—remain mysterious. Mr. Needham declines to disclose how many employees he has, or how many people make up the editorial team responsible for making changes. The Seattle-based site, which is owned by Inc. and gets more than 110 million unique users per month, according to comScore Inc., lists no address or phone number and doesn't disclose revenues.

The site's data-collection methods are at the center of a lawsuit filed by an actress named Junie Hoang, who has had roles in films such as last year's "Gingerdead Man 3: Saturday Night Cleaver." The suit, filed in U.S. district court in Seattle, seeks $1 million in damages. It claims that IMDb improperly exploited data associated with Ms. Hoang's credit card, which she used to register, to find out her birth date.

Unlike in some other cases, Ms. Hoang was miffed that the date, July 16, 1971, was accurate, not incorrect. She had been telling people that she was about 10 years younger. "Their defense is going to be that she was lying and shouldn't be rewarded for it," says her attorney, John W. Dozier. "Our position is that the [credit] information was only for the purpose of the transaction process."

Mr. Needham and a spokeswoman for Amazon declined to comment on the case. In a motion to dismiss the case, IMDb called Ms. Hoang's assertions "unsupported" and said that Ms. Hoang had voluntarily disclosed and permitted use of her subscriber information, per the site's terms.

Age has long been a touchy subject in Tinseltown. Actresses like Katharine Hepburn were known to deflect questions on the topic. Sandra Bullock admitted she'd been loose with the truth and "Beverly Hills, 90210" star Gabrielle Carteris famously lied about her age—she was 29—to nab the role of Andrea Zuckerman, a 16-year-old high-school student.

But as IMDb has grown in popularity, it is getting harder and harder for actors to fib.

Ryan Suratt, who had a small role in the 2009 remake of "Fame," spearheaded a petition to put pressure on IMDb to remove all birth dates from actors' profile pages. He has 5,010 signatures so far.

"I'm 30 now and can play anywhere from 25 to 33," insists Mr. Suratt. "I have a baby face."

The Screen Actors Guild has held meetings with IMDb to propose using an age range as opposed to specific digits. The idea would be to provide a "portrayable" range based on actors' looks rather than their actual date of birth.

Mr. Needham says he finds the notion preposterous. That actors care so much about their profile pages, he says, "validates the importance of IMDb to the whole entertainment industry."

The site dates back to the 1980s, when Mr. Needham, a self-described technology geek and cinephile, created a database listing the names of the cast and crew in all the films he had seen. "My family thought it was a crazy hobby that perhaps I might grow out of," he says.

In 1990, Mr. Needham published the database on a public discussion forum, luring contributors from around the world. Several years after moving it to a website, Amazon founder Jeff Bezos offered to buy the site in a cash-and-stock deal, the value of which has never been disclosed.

Some have found that it is easier to update a screen credit than to change other information, such as one's age. For that, the burden of proof is on the individual; IMDb says it requires evidence such as a passport to amend birth dates.

Upon reading of Ms. Hoang's travails, Mr. Lesser, whose own age has been corrected on the site, decided to take action. He penned an open letter to IMDb that was published in the popular industry trade magazine The Hollywood Reporter.

"Actors should only have to discuss their age in terms of range; in my case, I would say my range is between 45 and 70," wrote Mr. Lesser. In the meantime, he has appealed to IMDb to trim a few more years off his actual age.

Write to Rachel Dodes at