PORTLAND – Mike Cullinan broke up with his girlfriend and started a new life. Part of that new life included buying a big Harley-Davidson motorcycle: a 620-pound Dyna Low Rider with a 1,450 cc, fuel-injected engine.

At 38, Cullinan was undergoing just the sort of transformation that happens to many men approaching middle age: empty nest, divorce or just plain old midlife crisis.

Those riders, mostly baby boomers, are driving up the number of motorcycle registrations nationwide.

They also make up the fastest-growing segment of motorcycle deaths.

Across the country, the number of motorcycle fatalities among riders in the 40-plus category has jumped 200 percent over the past decade, while deaths among riders under 30 actually dropped, according to the National Highway Traffic Safety Administration.

“It’s really kind of astonishing: The ages of these fatalities are so high. You would think it would be all of the young kids on those fast bikes, but it’s not,” said Carl Hallman, highway safety coordinator with the Maine Department of Public Safety.

According to NHTSA’s National Center for Statistics and Analysis, the mean age of motorcyclists killed rose from 32 in 1994 to 38 in 2003. The surge in deaths among older riders helped to push motorcycle fatalities higher overall. They jumped by nearly half during the past five years, from 2,483 in 1999 to 3,661 in 2003.

In Maine, 22 people were killed on motorcycles in 2004, the highest level in a decade. The numbers are even more dramatic in Vermont and New Hampshire.

In New Hampshire, 29 died, compared to nine the year before. In Vermont, there were 11 fatal crashes, more than in the three previous years combined. In all three states, riders in their 30s and older accounted for the most crashes.

Safety experts say many older riders are either returning to the sport after a lengthy absence or have no prior experience.

“From a career standpoint, they have a little extra time and a little extra disposable income. The kids have grown up, so they’re looking for hobbies,” said Rae Tyson, a NHTSA spokesman who specializes in motorcycle safety.

That fits with the experience of many instructors.

“I’ve seen a definite increase in men in their 40s and 50s getting back into motorcycling. They haven’t ridden in 20 or 30 years, so their skills are rusty. Motorcycles have changed, and they’re getting bigger motorcycles. And they’re getting on without a refresher course,” said Cathy Rimm, program director for Motorcycle Rider Education of Maine.

No one knows exactly why baby boomers are dying at a faster clip on motorcycles because there hasn’t been a national study of the causes of motorcycle accidents since the late 1970s, Tyson said. Motorcycle groups are pressing for an update.

Big, powerful bikes appear to be part of the explanation. NHTSA data show that both engine displacement and fatalities among riders with the largest class of engines rose during the past decade.

Safety officials say older riders thinking about hitting the open road should recognize that their eyesight and reflexes aren’t what they once were. They also say the importance of a refresher course cannot be overstated.

“In our experienced-rider courses, we do take into account the way your body changes, that your reaction time will change and that your eyesight will change. There are changes older riders should make,” said Mike Mount, spokesman for the Motorcycle Safety Foundation in Irvine, Calif.

Though Maine and many other states require classes or clinics for new riders to get motorcycle licenses, there are no such requirements for a license holder who decides to get on a bike for the first time in decades.