I don't think 6 would necessarily be too young. My daughter read the first one at school at 7. DD reread it when our family listened to an audio book version during a recent roadtrip. I really enjoyed it - and the audio version we listened to was excellent.

I am tempted to make a "Pround Member of the Mysterious Benedict Society" teeshirt for dd on CafePress - in Braille of course!

Warning: sleep deprived