SO HERE IT IS

, the beginning of January, and right in front of us lies the perfect Caribbean clich : The sun sparkles on the pool as a calypso band plays and waiters circulate trays of rum punch. The air is full of the gentle clinking of Corona bottles and the murmur of lounging couples comparing the day's tan lines. But one sound is conspicuously missing here at the Renaissance Aruba Resort & Casino's Marina Hotel the pitter-patter of little feet.

There's a reason for that. Starting this year, not just the pool but the hotel itself was declared off-limits for the underage set. In fact, the entire resort has been split in two, with its pools, beaches and rooms divvied up between families and child-free adults like so many slices of pie. The adults come out of the deal with the glitzy Marina Hotel, its fitness room, the spa and even Flamingo Beach, part of the hotel's private island. Families, on the other hand, get the more low-profile Renaissance Ocean Suites, where perks like the Marina's infinity pool and martini menu have been replaced by swing sets and barbecue buffets.

You don't have to go to Aruba to find this latest travel trend. In a stunning reversal of the family-friendly travel movement, hotels are starting to take the opposite approach when it comes to children: banning them. While few have gone as far as the Renaissance, hotels from Rancho Las Palmas Resort & Spa in Rancho Mirage, Calif., to Vermont's Topnotch Resort & Spa are adding adults-only amenities. Even Atlantis, Bahamian granddaddy of the family-friendly movement, is adding a sleek new adult-oriented hotel, complete with kid-free pool and bar areas. Visitors to Cambridge Beaches in Bermuda, meanwhile, are welcome to bring kids under five provided they have a full-time nanny in tow.

What's more, hotels that have had age restrictions all along are tightening them up. At Little Palm Island Resort and Spa in the Florida Keys, the legal age has been bumped from 12 to 16. At Madrona Manor in Healdsburg, Calif., kids got a trial run when the resort opened, but now the place has just three hard-and-fast rules, says General Manager Joseph Hadley: No smoking, no pets and no kids under 12. Even rich and famous parents can't catch a break. When Gwyneth Paltrow asked if she could bring her daughter, Apple, to the 16-and-over Paraiso De La Bonita in Puerto Morelos, Mexico, she was told yes but only if she wanted to reserve the entire property for about $90,000 a night.

Hotels say the kid-free crusade simply makes sense because adults-only trips now make up nearly 70% of leisure travel while once-hot family travel is cooling. Demographics play a role here: Boomers, who spend $157 billion a year vacationing, are getting older, and as their children start families of their own, they're back to traveling deux. It's easier logistically, too, since hotels don't have to worry about maintaining pricey kids' clubs. Of course, money is the real issue. After all, kids don't run up the big bills that keep hotels in the black; they eat less, they share rooms, and they certainly don't order $15 martinis and $200 rubdowns.

Hotels say they're getting a great response to the movement, but not all travelers are embracing it. With all the focus on adding sophisticated perks for the kid-free set, some moms and dads are starting to feel as if they've been banished to the vacation version of Romper Room. And families with teenagers feel trapped in the middle, with children too old for the kiddie pool but too young for the bar scene. Even the hotels themselves are facing a slew of unexpected problems, everything from how to enforce age limitations without offending guests to keeping families from feeling like second-class citizens.

Catering to kids was once one of the hottest trends in travel. It all started with Mickey, of course, with the success of Disney vacations giving rise to megaresorts like Atlantis and Beaches Resorts, which wowed children with towering waterslides and jungle gyms while offering parents a stress-free vacation. Luxury hotels joined in too, with the Four Seasons rolling out a "teen concierge" and The Ritz-Carlton in Naples, Fla., launching its "V.I.K.," or Very Important Kid, program, complete with a miniature check-in desk.

But for many travelers, the pro-kid movement went too far. Hotels found children's programs too costly or had such low turnout for them that they were forced to combine children from toddlers to 'tweens in a single activity. The Mount Washington Resort in Bretton Woods, N.H., recently closed its teen center because staffing it became too expensive. Other hotels began charging guests extra for kids' activities, but even some of the more successful programs just managed to break even. And with youngsters an increasingly visible (not to mention audible) presence at the swankiest hotels, some adult travelers began to feel as if they were trapped in Neverland.

WHEN WE ARRIVE AT the Renaissance on a balmy Saturday, it seems the new regime is running a little behind schedule. The first thing we see is a maintenance man putting up the first of the new "Adult Exclusive" signs outside the Marina Hotel's pool. In fact, the Renaissance is one of a small number of hotels trying to have it both ways. By cutting the resort in two, the hotel hopes to woo more adults without giving up any of its family business, says Janien Huistra, director of sales and marketing. To play up its new split personality, the resort plans to roll out new packages aimed at each market such as the recent "Sexy and Sultry" package for adults: five nights, a $50 cocktail credit and a "hangover breakfast."

The hotel is scant on details, but Huistra says the plan has been "very successful" so far. Rooms here start at $350 at the Marina and $450 for the Ocean Suites; the property began to see an increase in adults-only bookings as soon as it made subtle, adult-friendly changes like reducing the maximum occupancy per room from four people to two. The adults-only packages marketed on the Web, meanwhile, exceeded expectations. "That's how we got confirmation there was an adult market for us," says marketing manager Rebecca Verschoor.

But some of the plans aren't going over so well with the resort's guests. We found Melanie Barnett on the beach keeping one eye on her seven-year-old daughter, Baylee, something she'd been doing a lot more of on this visit. A nurse from Santa Fe, N.M., Barnett spent two weeks at the Renaissance last winter and fell into a nice routine she'd work out while Baylee ate ice cream by the pool but this year she was told her daughter was no longer allowed by the pool at all. The staff made an exception, but said the ice cream would have to wait until the adults were served. (The hotel says the staff is easing into the new policy and in the future no exceptions will be made.)

Not surprisingly, when hotels divvy up the property, it's the kid-free adults who get custody of the more luxurious amenities like gyms, spas and high-end restaurants, leaving some parents feeling left out in the cold. Though the Ocean Suites has no gym, parents can visit the smaller fitness room on the private island, says Verschoor, though they'll have to take a boat there and back. The Ocean Suites also lacks for dining options: Its Captain's Corner restaurant serves dinner just twice a week, all-you-can eat buffets. The Marina Hotel's Blue bar, on the other hand, features sushi, tapas and five pages of cocktails.

Still, none of this is enough to dampen the enthusiasm of those adults who are thrilled to get a break. Lisa Zolga, a jewelry sales representative from Raynham, Mass., who had just checked in, was happy to leave her four kids at home. "Personally, I think it's great," she says over a rum punch at the hotel's happy hour. "There are two different kinds of vacations: kid vacations and grown-up vacations. And this," she says, gesturing at the Blue bar's hopping scene, "is the grown-up kind."