"Charles Darwin, aged 51."
According to the son of Charles Darwin, Francis Darwin, this portrait is by Messrs. Maull and Fox^[1]; he writes that "the date of the photograph is probably 1854; it is, however, impossible to be certain on this point, the books of Messrs. Henry Maull and John Fox having been destroyed by fire."

A photo like this one has been used in http://commons.wikimedia.org/wiki/File:CharlesDarwinHuntingSnark.jpg