CALGARY, FEB. 11 -- For most people, turning 25 is a big deal: A quarter-century, third of a lifetime, and still young enough to appreciate a milestone.

So what will it mean to towheaded Josh Thompson when he turns 25 next Thursday, two days before his shot at Olympic gold in the 20-kilometer biathlon?

"It's of no consequence at all," he said with fire in his gray-blue eyes. "I have way too much else to worry about."

These are sober times for Thompson, who shocked the biathlon world last year by winning the silver medal in the 20K at the world championships at Lake Placid, N.Y., the first American to upend Europeans in the sport they invented.

He shocked himself, too, and learned a lesson. Suddenly, he was a small-scale hero. People demanded his life story. He soaked up the glory so assiduously that he neglected training, and two days later bombed to 16th in the 10K.

The world is milling around again, but now he is tuning it out, something he does very well.

This is, after all, a man who grew up largely in western wildernesses, the only son of park rangers; who in his spare time from training learned to speak German, Norwegian and Italian and picked up his amateur airplane pilot's license.

"He doesn't have any idle time," said teammate Darrin Binning. "When he's done with his work, he opens his books and keeps on learning."

The studious Thompson is an unlikely looking world class athlete. At 5 feet 11 and 147 pounds, he's built narrow at the shoulders and broad and sturdy at the hips, and carries himself with the unassuming gait of a scientist who jogs.

But under that unimposing surface, said assistant coach Tracy Lamb, "is an aerobic animal."

Binning figures Thompson is skiing and shooting so well these days, "he's the man to fear" in the 20-kilometer Olympic event Feb. 20, and could be a force in the 10K three days later.

But biathlon, which combines cross country skiing with target-shooting, is unpredictable. "Bad days are easy," said Lyle Nelson, 39, the senior member of the U.S. team. "You miss three shots by a combined total of a half-inch and you're out of it."

Shooting accuracy is the lurking demon for biathletes, who stop four times in a 20K race to squeeze off five .22-caliber rounds through open sights at targets 50 meters away. The targets for shooting prone are about the size of a half-dollar; for standing, about the size of a coffee-can lid.

Penalties for missing are skiing laps, and chances are a medal winner won't miss more than once, Nelson said. The challenge, which makes biathlon appealing to the intellectually oriented, is the quick transformation from heart-pounding speed skiing to calm, controlled shooting.

"It boils down to knowing yourself," he twice said at a news conference here he'd rather not have attended. "The problem is to slow down {the pulse rate} and hit the targets. Everyone has a unique solution. There's nothing else like it."

More than anything, he said, success for him involves planning ahead, expecting surprises and knowing how to deal with them when they occur so he doesn't get flustered. Crosswinds, unusual cold, minor health problems, changes in snow conditions -- a thousand little details add up to success or failure.

For Thompson, a biology graduate of Western State College of Gunnison, Colo., who took up biathlon five years ago, the key to triumph here is good shooting. "He's skiing fast," said Binning. And Thompson loves the small, steep hills of the Calgary course.

"It's not a course you can win on just because you're fit, because the steepness means you need technique," Thompson said.

With international success already under his belt, he is enjoying his role as a competitor to be feared on a team to be respected.

He credits the infusion of funds from the 1984 Los Angeles Games with putting the biathlon program on track. He also got help from his home town where $10,000 was raised to support him in his last year of training in a program informally dubbed, "Send a Beggar to the Olympics."

He hopes to head back to Gunnison with something to show for it besides a birthday. A little gold or silver, perhaps, for a silver anniversary.