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'?
(..) with mother. She said nothing... Benjamin? Things were becoming different for me. My hair had very little gray and grew like weeds. My sense of smell was keener, my hearing more acute. I could walk further and faster. While everybody else was aging, (..)2008 The Curious Case of Benjamin Button
(..) here rest himself. Even like a man new haled from the rack. So fare my limbs with long imprisonment. And these gray locks, the pursuivants of death, argue the end of Edmund Mortimer." You thought I was plain ignorant, didn't you? (..)2008 The Curious Case of Benjamin Button