I'm not sure if it's all about talent either. Many of those weak, older players started when they were older, or weren't exposed to a lot of chess (or chess training) when they were young, while the "talented" kids often did start young and got training or at least were surrounded by many other chess-playing kids. I think that makes a big difference.
It seems that there's a critical age when one's mind is better able to assimilate and make sense of the patterns in chess, and to memorize moves and positions. Chess requires a lot of pattern recognition and memorization, and if one does not train those early enough, it may be too late.
It would be a very interesting study to find out how many of those old, weak players actually did have a lot of exposure to and training in chess when they were young.. my bet is not a lot.
Also, I wouldn't expect to find a lot of highly achieving chess players who started playing chess later in life (let's say past their teens), no matter how brilliant and talented they are in other fields, or how well they perform on IQ tests or SATs. They'll probably be beaten by much younger players, no matter how much time and effort they put in to chess.
Learning to play chess well seems to be somewhat similar to becoming fluent in a foreign language, which children also usually excel in compared to adults.