Young Adult literature is marked as YA these days to help parents figure out what should be appropriate.
Don't over sensor you child's reading choices. Ask them what they thought was interesting about the books they chose.
From Wiki...
Young Adult [YA] Literature uses a wide array of themes in order to appeal to a wide variety of adolescent readers. Some of these themes include: identity, sexuality, science fiction, depression, suicide, drug abuse, alcohol abuse, familial struggles, bullying, and numerous others. Some issues discussed in Young Adult Literature include: friendship, love, race, money, divorce, relationships within families...
Reading about issues that adolescents can relate to allows them to identify with a particular character, and creates a sense of security when experiencing something that is going on within their lives... Reading about it may help a young person validate his or her own experience and make some kind of meaning out of it.
The above noted, Lord of the Flies and Catcher in the Rye, are noted, these days, as more Adult literature than Young Adult.
The Crucible and of Mice and Men are noted as being YA.
The Hitchhiker's Guide to the Galaxy is YA 13+. People think you need to be a little bit older to get the flood of jokes. I don't think there is anything wrong with a 10 year old reading, at least the four books of the trilogy, these short, fun books. They will love the later parts about mice and dolphins being the species that actually run the world.