As long as the content isn't completely inappropriate or out of sync with your values, be happy they're reading. Overlook the faint glow coming from the room after lights-out. It's a great question for starting a lively debate. It's a bad one for getting a unanimous answer. At the other, some say it's natural to start reading at 9 or 10, and forcing it earlier is damaging.
Most people fall somewhere in between.
Problems arise when children's development doesn't match what's expected in their environment. At some private schools, strong prereading skills will be needed by age 4 for admission, and reading is expected by the end of kindergarten. Some home-schoolers say their individualized instruction lets children read when they're ready. One child can start reading well at 5, another at 9, without anyone being pressured or bored. At most public schools, children are expected to read in the early elementary grades.