things that will increase your risk of a heart attack would be high blood pressure, high cholesterol, low activity, being overweight, and smoking.
as for your second question about if people in their late teens/early twenties are likely to have heart attacks: no, it's not likely that you'll have a heart attack at that age BUT it's not impossible. at the canadian secondary school rowing association championship (aka schoolboys) there was either a pair or a double that won the final. when they finished, one of the guys reached over to high five his buddy, and he had died of a heart attack. when something like that happens it's usually because of a heart defect. this guy was in great shape and had been training very intensely for a long time but the strain that the race put on his heart was too much (your heart is running at max rate, or at least it should be if you're pulling hard, for around 7mins straight) and it stopped.
so it's unlikely but it is a possibility, especially if you have a heart defect and one or more risk factors mentioned above.