His books are enjoyable, but they don't do much to challenge the reader's 'gray matter'. [='intelligence']
His hair is 'graying'. = He has 'graying' hair.
What will you do when you are old and 'gray'?
(..) We're too late. Where are they going? You can't give up now! Thorin. You can't give up now. "Stand by the gray stone... ...when the thrush knocks. " The setting sun. And "the last light of Durin's Day will shine... " The last light. (..)2013 The Hobbit: The Desolation of Smaug