If you’re a 20-, 30-, or 40-something, you’re probably not too worried about having a heart attack. After all, only older, overweight people are at risk, right? Er, wrong. In fact, according to research from Harvard Medical School, anywhere from four to 10 percent of the approximately 735,000 heart attacks that occur annually happen to those under 45. That number is comprised mostly of men, but guys aren’t the only ones who should be nervous. Heart disease (which can lead to a…