I've read that they are quite common (about 1 in 50 people), but fortunately most never rupture. Fatal aneurysms in children are really rare though. Still 50-100 American children die of them every year. In adults, it's tens of thousands.
My sister had one that ruptured that would normally give her a 50/50 chance of making it, with anywhere from fine to vegetable.
She is one of the miracle ones. She survived it and has no residual issues.
My best friend and her kids found out when a doctor found something unusual in a scan and suggested they get the whole family checked. Theirs is genetic and they have all had surgery to deal with the ones that posed a risk. They were fortunate in that none of them ruptured.