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.

Make no mistake, 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.

Whatever the cause or the age, anyone with stroke symptoms needs emergency care: Sudden numbness or weakness in the face, arm or leg, especially on one side; sudden difficulty speaking or understanding speech; trouble seeing or walking; a sudden super-severe headache.

Younger adults are less likely than seniors to know those symptoms, and tend to try to shrug them off, Bushnell says. She points to a recent 50-something patient who twice ignored temporary episodes of weakness on one side. Called TIAs, for transient ischemic attacks, such episodes are a big red flag that a full-fledged stroke may be imminent. A third TIA finally brought him to the emergency room. By then, aggressive treatment wasn't enough to avoid a stroke that left him with impaired speech.

"As people get older, they have more and more direct contact with people who had strokes," and learn what to watch for, Bushnell says. But at younger ages, "there's just a gap in awareness."

Who is at increased risk for a younger-than-usual stroke? African-Americans and Hispanics, more than whites. Someone whose parent had a stroke before age 65 is at extra risk.

But mostly, the same things that are bad for your heart are bad for your brain, making it crucial to control blood pressure, diabetes and cholesterol, to stop smoking and to keep active. At www.powertoendstroke.org the American Heart Association offers a seven-step online test called "My Life Check" that can help assess your risks.

Younger people do tend to survive strokes more than older people, and to recover better.

But Arnold Springs, 48, of Winston-Salem, N.C., knows it was his friends' fast 911 call that made the difference for him earlier this month.

"All of a sudden, my right arm went numb. The next thing I knew I was on the floor," Springs recalls.

The ambulance got him to Wake Forest Baptist Medical Center in time for a clot-busting drug to stop his stroke. Springs left the hospital three days later with some loss of vision and trouble walking, problems that his sister says are expected to improve - plus orders to lower his blood pressure to stave off future strokes.

Lauran Neergaard covers health and medical issues for The Associated Press in Washington.