Of course it's for everyone! 25% of adults read them on their own or for they children.
However, MG novels are mostly written for readers age 7 to 12 years old with accessible events and voices that speak directly to this age group.
The hero reacts to external situations and events. The stories are full of adventures and many heroes travel around the world or through time.
The Magic Tree House by Mary Pope Osborne (1992 & on)
The 39 Clues. Collaborative authors (2008 & on)
11,000 Years Lost, by Peni R. Griffin (2004)
Cat in the Mirror, by Mary Stolz (1975)
Abracadabra Tut, by Page McBriar (2014)
The Iron Empire, by James Dashner (2014)
The Green Bronze Mirror, by Lynne Ellison (1966)
Running Out of Time, by Elizabeth Levy (1980)
The Mirror of Fire and Dreaming, by Chitra Banerjee Divakaruni (2005)
A Stitch in Time, by Penelope Lively (1976)
A Different Day, a Different Destiny, by Annette Laing (2009)
Horten's Miraculous Mechanisms, by Lissa Evans (2012)
Mira's Diary: Lost in Paris, by Marissa Moss (2012)
They tend to be funny and being hilarious sounds like one of the requirements to write MG novels.
Diary of the Wimpy Kid by Jeff Kinney (2012)
Timmy Failure- Mistakes were Made by Srephan Pastis (2013)
Charlie Joe Jackson's Guide to Not Reading by Tommy Greenwald (2011)
Milo: Sticky Notes and Brain Freeze by Alan Silerberg (2010)
The Adventures of BeanBoy by Lisa Harkrader (2012)
The Tapper Twins Go to War by Geoff Rodkey (2015)
So humor and adventure/culture seem to be at the heart of many Middle Grade novels.
There is so little time spent in the characters’ heads that many MG books are written in the third person (he/she).
Subplots tend to be absent or simplified.
Parents, siblings and family life are important. Belonging to or being part of the world/ a community/ a group is a recurring theme.
Vocabulary and sentence structure tend to be simpler.
These novels range between 20-000 and 40,000 words or well under 200 pages mostly.
(Note that there are exceptions. For example, the first book in the Magyk series by Angie Sage counts 564 pages.)
Upper MG novels are longer books that sound like young adult in subjects and storytelling, but they still are very young in themes and they stay away from edgy subjects.
Charlotte's Library. "Time Travel Books."
Lo, Malinda. "An Introduction to Middle Grade and Young Adult Fiction, Part I: definitions."