When a search index is generated, it includes entries tagged with the readability of documents referenced by the search index. A determination of the readability is made if a document is not yet associated with a preexisting entry in the search index and is being added, by analyzing the document to produce a readability indicator. For example, the readability indicator can correspond to a grade level identifier, thereby enabling search index to be searchable by grade level, age, or age range. A grade level-based search request received from a user can be submitted to search this search index for documents limited to a specific grade or age readability. The search results that are returned are thus filtered in regard to the readability indicator associated with the documents in the search index.