Being "declared" (excommunicated) means that no Scientologist is allowed to communicate with, or help, her in any way, under penalty of also being declared.
What this effectively means, for a girl in the Sea Org who was raised in Scientology, is being cut off from everybody she's known, and being forced out on the street, no money, and no idea of how to function in the "real" world. The psychological pressure is intense.
As to how these kids got into the Sea Org at a young age, kids of Scientologists are given recruiting talks at a young age. Parents who balk at having their kids talked to are given a very hard time.
L. Ron Hubbard went to see in the first place to do things untoward in international waters. From that insidious way around the law, all these other sea references came into being. To begin with, it was outer space and spaceships but yachts will do when you have an itch or two.