Can you be born with a brain aneurysm even if there is no recent history ruptured aneurysms in family? What causes it in a 20 year old?

Aneurysms congenital. Brain aneurysms are thought to be congenital and vast majority of them occur without any familial tendencies. However, some families do show multiple family members with aneurysms. Aneurysms are likely weak spot in the artery and grow slowly over years. It is unlikely to show up by 20 but it can.