What are the chances of a 21 yr old having a avm or vessels narrowing in brain?

<1% for brain AVM. Cerebral avms occur in less than 1% of the general population. These are congenital lesions that are often found incidentally when a ct or MRI scan is done for other reasons. If there is a family history of a brain avm, the chances are higher for having one. If there are symptoms such as seizures, headaches, motor weakness, tingling, an evaluation is warranted.
AVM in brain. Very small, less than 1 in 1, 000, but it happens. In most of the people who have them, they don't know it because they're not having any symptoms. These are the kinds of things the mris discover when they are performed for some other reason like headaches, or skull injury. If they're not causing any symptoms, don't worry about them. Treatmen(surgery) can be worse than the condition.