When you are building a collection and especially if you need to defend your collection against challenges, it is important to take into account the developmental needs of your user base. This is especially important at the school library, where discussions about what is or is not appropriate can become heated.
When considering the developmental appropriateness of materials in my collections, here are the resources I use:
Stages of Literary Appreciation from Literature for Today’s Young Adults by Alleen Pace Nilsen, et al. (PDF of first chapter provided by the publisher) Nilsen and her colleagues identify seven stages of literary appreciation, from birth through adulthood. When using this to build or defend your collection, it is important to remember that we retain characteristics from the earlier stages as we grow into the later ones. For example, in late elementary school, we may want to lose ourselves in the fantasy of literature. In middle school, we may want to find ourselves reflected in the books we read. Even though we now want to find reflections of ourselves, our desire for escape and fantasy has not disappeared.
The American Academy of Child and Adolescent Psychiatry Facts for Families. The AACAP has created an excellent set of resources on a variety of topics of interest to the families of children and adolescents. At their website you can download a complete set of these resources, search them by keyword, or browse them by keyword or in the order in which they were released. Two I have found especially helpful are Normal Adolescent Development - Middle School and Early High School Years and Normal Adolescent Development - Late High School - Years and Beyond.
Developmental Tasks and Education by Robert J. Havighurst. Havighurst identifies six stages of development and tasks that occur within them. The quickest overview of these is at Wikipedia.
Developmental Assets from the Search Institute. For a variety of age groups, the Search Institute has identified 40 developmental assets. These assets describe what children and young adults need to be successful and to avoid high-risk behaviors. While the other resources have identified characteristics of your students, these identify resources that enhance their lives. This can be useful for advocacy more generally and for selecting books where characters have and benefit from the developmental assets or do not have them and must work to overcome their situation.
Having these resources available makes it easy to justify the inclusion of works in your collection without having to rely exclusively on your personal opinion or even your professional judgment.
Many thanks to Sandra Hughes-Hassell for introducing me to these resources in her Young Adult Literature and Related Materials course.