I've previously written an article in depth on this. Basically, in short I feel it has a lot to do with how we absorb much more at a younger age, mostly due to how carefree we are and how less rules are self-imposed. As we grow older the task requires us to break down some of these blockers that prevents us for growing in new ways. It is definitely possible but it requires time and change.
http://maddchess.blogspot.com/2014/01/learning-at-older-age.html
Okay so this may seem as some what of an arbitrary question, but what are your opinions on the potentail progress of a player that starts seriously studying and playing past 20 or so. I know that there are clearly no specific or exact answers but what are some of your personal opinions. Keeping in mind that most, if not all, of the greats started playing and training very young.