"We love to watch the world pass by," says Carl.

Demographers predict that over the next 20 to 30 years, Philadelphia will see a steady growth of folks like the Schneiders, an over-50 population ranging from "pre-seniors" who may still be working full-time to the elderly elderly. They will leave the suburbs for the convenience of being able to walk nearly everywhere, the comfort of nearby neighbors, the stimulation of the city's cultural institutions and the proximity of good hospitals.

Currently, close to 30 percent of Center City's population is married couples with no children, and empty nesters are the fastest-growing segment, according to the Center City District.

The Schneiders are each 75. In March, they will celebrate their 50th anniversary. Forty-seven of those years were spent in Elkins Park. Quintessential suburbanites. Raised four kids. Mowed the lawn. Clocked enough miles on the cars to get to Pluto and back, mostly within a 20-mile radius.

"We were very domesticated," said Carl, former chairman of the corporate law department of Wolf, Block, Schorr & Solis-Cohen.

But three of their four children are in Center City. So is the theater. Ballet. Opera. Restaurants. So in the spring of 2005, they moved to Center City. One of the glam condos that are shooting up in Philadelphia like gold-leafed sunflowers. Just the right perch for a couple with another good 10 or 15 years left to get their urban groove on.

The baby boomers are right behind.

"You definitely see this trend," said Anthony Flint of the Lincoln Institute Land Policy, a think tank in Cambridge, Mass. "There's no question. Empty nesters find themselves out in a more isolated suburban setting. The kids are gone. They don't want to have to clean the gutters. Moving into an urban setting just makes an awful lot of sense."

In all likelihood, however, they are more likely to arrive in flocks nationally than Hitchcockian multitudes. In part, that's because city living can be expensive. But mostly, analysts say, it's because of the emotional bond to the familiar - and the inherent human desire to hold on to stuff.

"It was traumatic to downsize," said Nora Dowd Eisenhower, Pennsylvania's secretary of aging. "To go through everything you own and say yea and nay." She and her husband, Jim, traded in their Chestnut Hill house for a Center City apartment this year after their daughter left for college. "I talk to a lot of people who are older than us, grappling with it. The emotional hurdle is so great, they go up to the attic, sit there for five minutes, and say forget it."

In a study published in the spring, "Mapping the Growth of Older America," William Frey, a demographer at the Brookings Institution, reported: "As of yet, there is little evidence of a widespread 'back to the city' movement nationwide among older populations."

Still, if only a small percentage of the boom finds the fortitude to take the plunge, that will make for a lot more ticket sales at the Ritz, bagels and lox at Di Bruno's, and highlights at Richard Nicholas' salon.

Real estate agents say the process of cutting loose for empty nesters takes longer than for younger clients. And usually one spouse is more eager to move than the other.

"It usually starts with a phone call," said Center City agent Joanne Davidow. "They'll say, 'I want to look. We're not ready yet, but what are we doing living in this big house out in the suburbs?' "

That's how it began for Tom and Monica Teesdale. Both 58, with three grown children, they had lived in Blue Bell for 20 years. Tom, a West Point graduate retired from the Army, had grown up in Mayfair. Monica was from North Philadelphia.

"I was the Girl Scout cookie mom. The kids were heavily involved in sports. Soccer. Cross-country. Field hockey. Basketball." She worked at Plymouth Meeting Mall. Played tennis with friends. Volunteered.

It was a fulfilling life, but without sidewalks.

Then she and Tom visited their daughter Suzanne, who lived for a while in Buenos Aires, Argentina, and then Barcelona, Spain. "I realized how much I loved the city's energy and being able to walk everywhere," she said.

So after their youngest graduated from college in 2003, Monica proposed moving to Center City.

Tom wasn't against the idea.

But he was running a company that was keeping him very busy. "I had too much on my plate," he said.

In 2004, the glaucoma that he had been keeping under control for decades got worse. Over three months, he lost 95 percent of his vision. "I became a prisoner of my home. It was a nice home. I took piano lessons and put in exercise equipment. But I could not go out the door to take a walk. Sometimes, you wake up and say, 'I want to walk down the street and buy a cup of coffee.' "

To make matters worse, the family's Bouvier, Maddie, died. "I was thinking about getting another dog. Then I said to Monica, 'Are you still interested in moving into town?' "

It took 18 months for them to find the right place. In June, they moved to a large apartment in the Bank Building on Chestnut Street, attached through an underground passage to the Omni Hotel. Through a joint agreement with the developers, the hotel's concierge and valet service, and the gym, are available to the condo owners.

Now that they're settled, Monica is looking for a part-time job.

Tom has found another piano teacher. He listens to books on tape. And he's won his freedom.

A few weeks ago, he went out to pick up a pizza.

"Can you manage it with your stick?" Monica asked.

"We'll see," he said.

Twenty minutes later, he arrived at the door, jubilant, holding the pie.

The Teesdales, who grew up in working-class families, say they recognize that they are able to afford a city lifestyle that is out of reach for most people.

"It makes sense," said Dowd Eisenhower, "to consider tapping into what, for many people, is their only asset: their home. . . . And to ask yourself - now - what you want the rest of your life to look like, rather than when you're up against a crisis."

Urban planners say that as the baby boomers age, cities like Philadelphia are going to have to provide housing and services for empty nesters who want to spend their retirement downtown, but also for those who have never left.

"The baby boomers are very stratified into haves and have-nots," says Frey. "What cities have an eye on is people with a lot of money. But the have-nots are going to have to live somewhere, too."