It all depends on several factors. In the case of a written text or books, generally the volume (or the length) and the diction (style of language) should be taken into account.
At the elementary level, texts should not be too long, rather pictorial story-books are undoubtedly the best. Poems should desirably have regular rhyme patterns and rhythm.However, some poems are suitable for all ages. For example, The Daffodils by William Wordsworth, may be taught at elementary as well as university level.
For story books, the length and diction surely counts. However, the content need to be considered as well. Fairy tales, folk tales,funny stories, fables and the likes are generally accepted suitable for pre-teen ages (say, 5-12 years). But teenagers should be encouraged to read tales of adventure, humour, mystery, sci-fi, detective stories, as well as writings on scientific discoveries and inventions, nature, environment, real life dramas, (auto)biographies, etc. However, The same genres of books may be well-suited for adult readers if the style or diction is characterized by critical rhetoric or figures of speech. Philosophical and literary writings are prescribed for adult learners. For that reason, novels and articles may be classified as suitable for either teenagers or adults.
The same criteria are applicable for audio and video resources. Above all, it is the approach to the resource as well as the mode of presentation that determines the appropriateness of a resource for any particular age group.
Besides, a teacher might like to edit/ censor, at his/her discretion, certain words or expressions of a text/audio-visual resource to suit the target age-group.
For relevant resources on this topic, you may like to read more.