Board books are printed on heavy cardboard-like paper, so babies can chew on them. They are typically for newborn babies to ages 2-3.
Concept books are picture books that educate children about shapes, colors, numbers, and letters, among other subjects. There seems to be less consensus about the target age range, but parents typically read them to children ranging from newborn to preschool age.
Picture books are typically seen as ways to teach literacy, i.e. written for preschoolers and kindergarteners. Some picture books are, however, very lengthy. Some deal with difficult subjects such as prejudice or historical events like the fall of the World Trade Centers. As a result, picture books can be written for ages 4-8, though some teachers continue to use them in the classroom long past third grade.
Readers should also keep in mind that picture books often have high Lexile scores and can have larger vocabulary and more complex sentences than early readers. This is because they are meant to be read aloud by an adult.
Also called beginner readers, early readers, or leveled readers, these books are for children just starting to read on their own. As a result, they tend to feature short sentences and sometimes repetitive text. They are usually read by children roughly from 5-7. Some parents use the levels on the covers to find a suitable title, but the levels are not standardized across publishers. Examples include the Piggie & Elephant books, some Dr. Seuss books, and the Henry and Mudge books.
Contrary to popular belief, chapter books are not any book divided into chapters. This term refers to books for newly independent readers, usually around the ages of 7-9. They are shorter books like Magic Treehouse and the A-Z mysteries.
Lower Middle Grade
Typically, middle-grade is said to be for children ages 8-12. However, middle grade is understood by librarians, teachers, and even authors as typically being split between upper and lower categories. I would say that lower middle grade books are for ages 8-11, or roughly third to fifth grade. These books are usually shorter, simpler, and not as dark as upper middle grade books. However, they are longer and more complex than chapter books.
Upper Middle Grade
Upper middle grade actually spans a bit of a larger age range than is commonly acknowledged. Upper middle grade books are suitable for ages 11-14, or roughly sixth to eighth grade. Books such as Percy Jackson, Harry Potter, and Keeper of the Lost cities are upper middle-grade books and certainly appeal to readers older than 12.
Young adult books are supposed to be written for ages 13-18. However, until “lower” and “upper” YA categories are established, I would argue that most books being published as YA today would be more suitable for older teens, perhaps 15 or 16 to age 18 (and up).
Age ranges for different books can be defined differently by different individuals and institutions and, indeed, are generally more fluid than lists like this can indicate. This is simply my suggested guide for finding an age-appropriate title and for becoming more familiar with different terms (chapter books, for instance, are often conflated with middle grade). It is, of course, possible that any given child will be reading above or below the suggested age ranges–that is perfectly normal and perfectly okay! Again, this is just a general guide, not meant to be a substitution for researching the appropriate books for any given situation and certainly not meant to be a means of keeping children (or adults!) from reading books they enjoy.