Fiction written for children between 8 to 12 years, with the most common range seeming to be 10 to 12 years.
There is wiggle room and plenty of exceptions. Kids as old as 13, 14 and 15 usually still enjoy some middle grade fiction as a nice read. Some middle grade fiction are not appropriate for an 8 year old to read individually, yet may be completely fine for an adult to read aloud to that same 8 year old.
Some adults also enjoy middle grade fiction (Harry Potter anyone?).