How can a young person get a stroke?

Several. Stroke in a young person can have many causes including abnormal blood clotting, blood vessel injury, a ruptured brain aneurysm or avm, severe high blood pressure, heart abnormalities and many other causes. The cause of the stroke should be investigated and treated in order to prevent future strokes.
Cryptogenic stroke. This is commonly referred to as a cryptogenic stroke meaning that the csuse is obscure. A stroke in a young person, especially one without the ususal risk factors, suggests a connection between the right and left sides of the heart. (an atrial septal defect). If this is present, a clot can get from the right side to the left and if it goes to the brain, a stroke can occur.