Heart disease is the leading cause of death in the United States, yet people generally associate it with an older, aging population. But heart attacks, strokes, and other types of heart disease can be blind to age — particularly when certain factors are in play.

Here are five reasons why heart disease can strike by the age of 50:

Smoking

One of the biggest risk factors for coronary artery disease among the young also is the only one that's completely preventable.

Young adults who smoke multiplied their chances of having a major heart attack over their non-smoking peers, according to various studies, including one that found it increased the risk by eight times as much.

"It's a very potent risk factor that's disproportionally present in younger patients," said Dr. Willie Lawrence, chief of cardiology at Research Medical Center in Kansas City, Missouri. "If you're a younger person who wants to avoid a heart attack, the most important thing you can do is not smoke."

Pregnancy complications

Women who develop preeclampsia, gestational diabetes, gain excessive weight, or go into preterm labor during pregnancy are at increased risk of early heart disease. That's because artery-related problems are usually at the core of pregnancy-exclusive conditions, said Dr. Maria Sophocles, medical director of Women's Healthcare at Princeton in New Jersey.

"Pregnant women who have these complications are really giving us a little window into what their future vascular situation will be," she said.

All women should talk to their doctor, or even a cardiologist, about heart-related concerns if they experience pregnancy-related problems, she said.

"Preterm delivery often happens because the vascular system of mother and baby is compromised. It's often because the blood vessels couldn't support the pregnancy any longer," Sophocles said. "So that's a hint that maybe her cardiovascular system is not going to be as squeaky clean for long term."

Familial hypercholesterolemia (FH)

High cholesterol is a factor for heart disease, but the genetic condition familial hypercholesterolemia (FH) is a special form of it. FH contributes to excessive levels — above 190 mg — of low density lipoprotein, or the "bad" LDL cholesterol. People born with FH have high cholesterol from a young age.

About 1 in 250 people have FH, which usually leads to problems associated with high cholesterol such as atherosclerosis. Half of men with untreated FH will have a heart attack or angina before they turn 50. Thirty percent of untreated women will have a heart attack before they turn 60.

"People with FH see artery disease progress earlier and more rapidly than people without FH," said Sophocles, who has the condition and is a board member on the FH Foundation.

Awareness is crucial, she said, but only 10 percent of all FH cases are diagnosed.

Race

Race and some risk factors combine to increase the dangers of heart disease. While race can't be changed, risk can be significantly lowered with effort.

High blood pressure is a primary risk factor in heart disease, and its prevalence in blacks in the United States is among the highest in the world.

About three out of four black adults develop high blood pressure by the time they're 55, according to a recently published study.

But Lawrence said making lifestyle changes such as getting regular exercise and eating healthier, particularly reducing sodium intake, can help manage high blood pressure. Recent studies have provided better guidance on options for lowering blood pressure, as well as what numbers to aim for.

"We now have a better sense of which medications that doctors should choose, and what blood pressure to target," he said.

Depression

A heavy heart affects the entire body, not just the mind. Depression releases stress hormones and increases inflammatory chemicals that can cause a narrowing of the arteries.

It also has an impact on lifestyle factors that may take a direct toll on physical health, said Dr. Gazala Parvin, a family physician and Integrative medicine specialist at Research Medical Center in Kansas City, Missouri.

"If you are depressed, it affects your lifestyle. You don't eat, so your diet is bad. You don't exercise. You don't sleep well, and you don't socialize," she said. "If mental health is not good, you are not going to take good care of yourself."

Teens who suffer from depression or bipolar disorder should be monitored for early heart disease, according to an American Heart Association scientific statement that found their mental illness puts them at early risk for heart and blood vessel disease.