DENVER — The ski slopes that baby boomers used to shred when they were young and reckless are being toned down in a bid to keep them coming back for more.

"Every destination resort that attracts baby boomers is either expanding grooming or contemplating doing it," said Bill Jensen, chief operating officer at Vail, which plans to flatten and tame a record 1,600 acres this season.

Demographics suggest the bottom line is behind the move: The percentage of skiers who are 45 or older climbed from 21 percent in 1997-98 to 31 percent last season, according to a survey done for the National Ski Areas Association.

America's seniors are so much fitter than their predecessors — retirement is more like a permanent vacation — that some resorts no longer offer free skiing to those over 70. But older skiers generally have slower reflexes, less strength and aching joints, and resorts are trying to respond.

Craig Tuber, 60, a Chicago resident who owns a second home near the Beaver Creek resort west of Vail, still occasionally skis bumps and even heliskis. But, he added, "when the grooming is great you get drawn to it so you can make those nice turns and enjoy yourself in a different way."

"For people who are a little older it is easier on your body," he said. "Once you turn 60, you say to yourself, 'How long can I do this?' Now you have a much more unlimited time horizon."

Every major resort prides itself on a variety of ski runs — beginner, intermediate and expert, with some also offering "extreme" terrain that is both steep and potentially full of unmarked hazards like tree stumps and rocks.

In the past, grooming usually applied largely to beginning and intermediate runs, with snow machines pulling wide metal "drags" that knocked down bumps and smoothed out irregularities in the slope. What's left is often called corduroy because of the distinctive grooves left in the snow.

But some resorts now routinely groom half an expert slope, offering the serious stuff on one side and an easy way down on the other. And the easy way down isn't for the kids.

"We are very aware of the importance of keeping this age group skiing," said Christine Horvath, spokeswoman for Squaw Valley in California. "We have been extremely focused on the quality and quantity of our grooming, especially during the past two seasons."

Mammoth Mountain, also in California, has found that after years of bumpy moguls and powder skiing, baby boomers love the corduroy because they can enjoy steeper slopes without putting as much stress on their bodies, said Pam Murphy, senior vice president of the resort.

At Vail, nine of its 29 snow machines are being replaced as part of the effort to boost grooming, Jensen said. It's not a small step, since each machine can cost as much as $235,000.

Beyond the slopes themselves, the evolution of the shaped ski is a big factor in keeping boomers skiing, Jensen said.

"Initiating a turn has never been easier — but the reality is that any ski, straight or shaped, performs best on straight and groomed surfaces," he said.

Fitness experts say boomers need to be careful, even on groomed slopes.

"Many of the severe knee injuries occur on very forgiving, flat terrain that is well-groomed," said Dr. Robert H. Johnson, a professor of musculoskeletal research at the University of Vermont and a specialist in skiing injuries.

Ski areas haven't totally forsaken the boomer who wants to shred. Aspen Mountain, for example, offers a "Bumps for Boomers" program designed to take skiers comfortable on groomed slopes to more difficult terrain using "techniques that both reduce fatigue and stress to aging joints."