Nice article, with a lot of truth in it. There are other aspects, as well: a) most adults simply do not have the time (not just in years) to put into chess as young people do, b) adults often have stronger ego issues attached to suffering through failure, and c) many adults begin to value the pastime aspects of the game more than the competitive aspects - which tends to diminish the ambition and aggressiveness that often propels younger players to achieve.
There is a good book out there entitled Master at Any Age. This book outlines how one older person grasped the title.
Maybe at some point, I'll wax more on my own experience.
Added a new blog post on the challenges adults face when learning chess, and what they can do to work around some of their experiential luggage.
You can read the post here: http://www.beginchess.com/2009/07/05/challengesofanadultlearningchess/
And I would like to here some ideas from adult chess learners as to what they think some obstacles are, and what techniques they have used to overcome them.