It's normal to monitor your child's literacy benchmarks, and parents often spend a lot of time worrying about whether their children's reading skills are age-appropriate. Educational institutions put out a variety of "reading level" charts that parents can use to choose books in the right age range. However, these charts are intended to be suggested guides for parents, not prescriptions about exactly which books a child should be reading.
Children who are just starting to develop their reading skills will need books for beginning readers aged 0 to 3. These are very simple books that may not have a story at all, just a collection of words with associated images. If they do have stories, they will be very simple, without complex vocabulary. The focus is not on the narrative, but on getting kids to the point that they can spell out words and understand them within a sentence. Many of these will be printed as "board books," a special type of book with thick cardboard pages. These books are easier for very young children to hold, and can stand up to being dropped, tossed around, and chewed on. Parents and guardians can read more advanced books with their children at this age, but they usually need to hold the book and read it aloud.
As children develop their language and fine motor skills, they can move on to regular picture books. Books for 4-year-old will start off with big, colorful illustrations and a relatively simple story without too much text. These stories may have a basic plot and give the protagonist some obstacles, but they're still fairly basic narratives. As children age, the picture books they can tackle become more complex. They can handle more text in a story and they're better able to follow a tale with more conflict. Picture books for aoften involve the protagonist solving a problem or overcoming a challenging situation. By the time kids reach this reading level, they're old enough to understand sentences without sounding out each individual word.
Generally, by the age of 8, children are less reliant on pictures to grasp the meaning of a story. They can move on to books where pictures appear only occasionally, and many are ready to tackle all-text books. By the time they're 9 or 10, some can read surprisingly long and advanced books! At this stage of reading development, kids no longer need to have their parents read aloud to them, but it's still a good idea to have conversations with your child about the books they're reading. Some precocious readers may find that even if they're able to follow the plots of advanced books, they may struggle with difficult subject matter such as violence or the death of a beloved character in a story.
If your child is still interested in picture books past the age you believe it's "appropriate," don't panic. It's possible for parents to be a little too concerned about pushing their children toward challenging reading material. Children often return to familiar books beneath their reading level because they find comfort in returning to a story they know they enjoy. Kids also like to look at illustrations at all ages, and an artistic child might not get enough chances to explore the pictures they enjoy at school! For older kids who seem especially fond of picture books, consider picking up age-appropriate comics.