WASHINGTON – When a stroke hits at 52, like what happened to Sen. Mark Kirk of Illinois, the reaction is an astonished, “But he’s so young.”

The reality is that strokes don’t just happen to grandma. They can happen at any age, even to children – and they’re on the rise among the young and middle-aged.

That makes it crucial to know the warning signs no matter how old you are.

“Nobody’s invincible,” warns Dr. Ralph Sacco, a University of Miami neurologist and past president of the American Heart Association.

Every year, about 795,000 people in the U.S. have a stroke. While some strokes are caused by bleeding in the brain, most are like a clogged pipe. Called ischemic strokes, a clot blocks blood flow, starving brain cells to death unless that circulation is restored fast.

The vast majority of strokes do occur in older adults. But up to a quarter of them strike people younger than 65, Sacco says.

In the so-called stroke belt in the Southeast, that figure can be markedly worse. At Wake Forest Baptist Medical Center in North Carolina, a stunning 45 percent of stroke patients are young or middle-aged, says stroke center director Dr. Cheryl Bushnell.

More ominous, recent government research found that nationwide, hospitalization rates for ischemic strokes have jumped by about a third among people ages 15 to 44 over the past decade.

Sometimes younger-age strokes are flukes with no warning signs, impossible to predict – like Kirk’s appears to be. The Republican senator is a Navy Reserve commander and avid swimmer, but dizziness sent him to the hospital. It turns out he had a tear in the carotid artery in his neck which blocked blood flow to his brain, triggering a stroke.

Trauma usually causes such tears, although doctors haven’t been able to say what caused Kirk’s. His doctor at a Chicago hospital said Monday that Kirk was continuing to improve from the stroke, which affected his left side.

Heart birth defects, such as a little hole in the heart known as a PFO, and blood-clotting disorders also tend to cause strokes more often in younger people than in seniors.

But just like strokes at older ages, a lot of younger strokes are preventable. The increase seems to be part of a troubling trend: As Americans get fatter, high blood pressure, diabetes and other artery-corroding consequences set in at an earlier age – meaning resulting strokes can hit earlier, too.

Indeed, research reported in Annals of Neurology last fall found nearly 1 in 3 of the 15- to 34-year-olds hospitalized for a stroke, and over half of those ages 35 to 44, already had high blood pressure.

More women are having strokes during or right after pregnancy, too, the government reported last summer. That’s because more of them start out with unhealthy conditions like high blood pressure even before the hormonal changes kick in.