There's a lot of great books one could assign. Easy, right? Of all ages is the kicker! What's a great book that a college student and a first grader could read and get something out of?

Some of the short stories of Hemingway would be good (be careful because of some of the themes for the young students!).

Parts of the Meditations of Marcus Aurelius.

War of the Worlds by H.G. Wells. It's a very colorful and intellectual book but NOT difficult to read at all.

The Martian Chronicles by Ray Bradbury. It's a philosophical book but not terribly difficult.

Kidnapped by Robert Louis Stevenson. A classic with, let's face it, classic themes and even a sort of lilt to the writing style.

All of these books are sophisticated yet simple, aren't terribly long and are enormously interesting no matter what one's age or ability.

Finding good classic literature that works for students of all ages is difficult, as students reading novels generally go from ages 10 or 11 to 18. There are a few that, while they may not be particularly challenging for older students to read, will interest children of many age groups. I will list a few below.

To Kill a Mockingbird by Harper Lee is an excellent book that a person of any age could enjoy. It is primarily a coming of age story about the narrator, a young girl named Scout, who lives in small town in Alabama. Her father is a lawyer who defends a black man who was falsely accused of raping a white woman. The novel deals with equality, understanding and justice, appropriate lessons for any student.

Oliver Twist by Charles Dickens is another good read. It is also not as complex as many of his other books, allowing children of many ages to enjoy. It follows the story of the orphan Oliver and his struggles as he works to get out of the clutches of the conman and criminal Fagin.

The Once and Future King by T.H. White is a great story for any student interested in legends. While it is a bit long, White is a superb writer and can keep a reader of any age entertained. The book is the story of King Arthur from the time he is a young boy, and the rise and fall of Camelot.

A few others: Farenheit 451 by Ray Bradbury, The Adventures of Tom Sawyer by Mark Twain, The Adventures of Sherlock Holmes by Sir Arthur Conan Doyle, Little Women by Louisa May Alcott, Treasure Island by Robert Louis Stevenson, and The Hobbit by J.R.R. Tolkein. The link below gives a great list of classic literature suitable for children --

http://www.life123.com/parenting/education/children-reading/classic-literature-reading-list-for-middle-school-.shtml

Wow, students of ALL ages? That could be rather difficult to establish. There is such a huge difference between readers as they mature. Think of the maturity difference, for example, between a 14 year old freshman in high school and an 18 year old senior.
That said, at the high school level, works by Edgar Alan Poe are usually quite popular and short enough for students to engage. Some of the language can be hard for students to read. They are all available online, too. Short stories by O’Henry such as The Gift of the Magi and The Blackjack Bargainer also engage students well. Frank Stockton’s The Lady or the Tiger will give students much to discuss as they untangle his trick ending. All of these can be found online at http://bibliomania.com/.