Medieval History/Middle ages
is there a book at 7th grade reading level for someone who wants to understand how the Dark Ages started?
Seventh grade is a tough, in between reading level. Many books written for adults are written at an eighth grade level, so don't be afraid to try some books that are aimed at an older audience.
A quick internet search found these titles:
Saint Augustine: A Penguin Life by Garry Wills
How the Irish Saved Civilization: The Untold Story of Irelandís Heroic Role from the Fall of Rome to Rise of Medieval Europe by Thomas Cahill
If you like history on the light side, you could try "The Decline and Fall of Practically Everybody" by Will Cuppy. It contains many short tongue in cheek biographies including Attila the Hun, Charlemagne, Lady Godiva, and William the Conqueror as well as many other people outside your time frame.
You could also search on "Fall of the Roman Empire" since that and "Start of the Dark Ages" are almost interchangeable.
Hope this helps,