Recommended age range for books?

I haven’t seen this before, but note well if you’re looking to read Jack Reynold’s latest:

Chronopathologies: Time and Politics in Deleuze, Derrida, Analytic Philosophy, and Phenomenology [Hardcover]
Jack Reynolds (Author)
Reading level: Ages 22 and up
Hardcover: 292 pages
Publisher: Lexington Books (December 8, 2011)
Language: English
ISBN-10: 0739132814