If you're over 50, you've probably had this experience:

You're standing at a checkout counter, ready to pay, and the twentysomething behind the register is talking on her cell phone. So you wait, and wait, and wait, and when the clerk finally finishes her conversation, she offers not an apology, but a grimace that suggests you've interrupted.

Sound familiar? It has a name: the Service Gap.

That's not a hip clothing store for soldiers. Or a new motto for the London subway system.

It's business-speak to describe a phenomenon fueling plenty of holiday-shopping frustration: the difference in how baby boomers and members of the "millennial generation" define the concept of customer service.

"There's a tremendous culture and value gap," said William Withers, a communications professor at Wartburg College in Iowa who, with his colleague Patrick Langan, has spent years studying customer service.

It's not that those in their teens or 20s are lazy or mean. Or that boomers are getting crotchety as they age. It's that both groups have far different social experiences and expectations, and demographics tend to put boomers in the checkout line and millennials behind the counter.

By 2010, Withers predicts, boomers will constitute nearly half the population - and control about 65 percent of the disposable income. As shoppers, they were shaped in an era when department stores were well-staffed with experienced salespeople who greeted customers with a polite "How may I help you?"

The millennials, a cohort almost as large as the boomers and one that will continue to make up a substantial portion of the service sector, have no such experience. Their shopping background is more impersonal, often conducted via phone or the Internet. Even their social networks are maintained via technology - text messages, instant messages, and Web sites such as Facebook and MySpace.

To put it in words a boomer can understand, face-to-face interaction just isn't their bag.

"They're clueless," said Franni Segal, 55, a travel agent with a degree in retailing. "They're more interested in talking to their friends. They have no incentive to make the sale."

When she was in her 20s, working on commission as an assistant manager at Bonwit Teller, she recalled, she and her colleagues eagerly courted customers. "Today when you go into a store, they're not interested in your needs. . . . With the kind of service you get now, you might as well be on the Internet."

Neil Simpkins, a boomer who waited tables and tended bar as a young man, said he was appalled by what passed for service these days.

"It drives me crazy," said Simpkins, 53, an account supervisor for LevLane Public Relations in Center City.

Last week at Macy's, he was hurried and a little lost while searching for winter gloves. An older saleswoman approached and asked if she could help. "She walks me to the gloves and asks which size I need. I was out of there in two minutes. She didn't have a tip cup out, either."

He contrasted that encounter with visits to local nightspots where young bartenders continue to chat with coworkers rather than take his order.

But Sarah Collier, a 20-year-old Arcadia University junior who has worked in retail sales, said she believed her peers' customer-service skills depended on their background and training, not their age.

"I don't think it's necessarily the entire generation. I think it's just the individual," said Collier, a hard worker who now sells time-share vacations for a Poconos resort. "I know how to give good customer service."

Others do, too. Withers takes care to note that many excel at the craft, but said that, generally, "those who are coming up in the service sector, they're just another breed of cat. They have interpersonal skills that we're finding are underdeveloped."

Millennials are more likely to have been raised by single or divorced parents, and in families in which both parents worked. Their cultural touchstone isn't the brotherhood of Woodstock but the bloodshed of Columbine. As clerks and waiters, they can struggle to meet customers' expectations.

More is at stake than etiquette. The bottom line for a business is, well, the bottom line. And dissatisfied customers will spend their money elsewhere.

Others suggest the problem is one of age, not culture.

"I'm not seeing the Service Gap," said Susan Magee, an adjunct professor at Chestnut Hill College who has studied the millennial generation. "I just think they're kids."

Remember, she said, it's the holiday season, and store clerks of all ages are busy and stressed like everyone else. "I think at Christmas, if the person can manage to say, 'Hi,' get my item in the bag, and not forget my gift receipt, I'm satisfied."

Withers blames the Service Gap partly on employers: Many lack the time or money to train workers. A 2005 study written with Langan reported that 78 percent of companies provided "some" customer-service training, but that 68 percent wanted to offer more.

What's the answer for peeved holiday shoppers?

Adjust your expectations - down, Withers said. Or be willing to pay more to shop and dine at establishments that have the time, interest and money to train their employees.

Nobody should waste their breath berating a young clerk or waiter, he said.

"Calling over the manager isn't going to matter," Withers said. "The server may still not look you in face. And they'll be texting their friends about what an ass you are."

Contact staff writer Jeff Gammage at 610-313-8110 or jgammage@phillynews.com.

Inquirer staff writer Melissa Dribben contributed to this article.