Basically, some books are so charming, even older teens and adults may enjoy them. In addition, if a person has a reading disorder or language barrier, a simple and yet wonderfully written book may encourage him/her to read other books by that author or in that genre.
On the other hand, some readers are already enjoying books that are far beyond the targeted age range. So I make a few arbritrary decisions on books that may be perfect for a teen that reads at an 18+ level.
Remember, it's not a cliche, age really is just a number.