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.
Many amateurs never get good because they they don't practice the right things(e.g. putting and short game). Drive for show, putt for dough.
Golf seems to be fairly similar. Lots of players spend every weekend at the range and take endless lessons and workshops, but never manage to get their handicap down to single figures.