The top FIDE rated players (2700 chess) are all under 50 years old. Most of them seem to be around the 23-32 age range. Why is this the case? I don't really know if this has always been the case or if it this is just something apparent in the current generation.
I'm guessing this is the case for a number of reasons that may include:
Well, all you mentioned are probably the reasons for some players.
I would also add 'lack of time' as one of the reasons. When you get older you may need to go to a college/university (chess don't bring much money, unless you are at the top), then family life/kids (chess players have to travel regularly a lot) makes travel harder... So, many people 'switch' to teaching and playing only in local tournaments.
It may be possible for an older man to "maintain" a certain level of play, but people's ability to improve and learn new things peak in their early 30s.
Chess is a dynamic and changing game. Someone who had trouble keeping up with the latest developments would fall behind, even with the same level of raw ability. That describes most people after, as opposed to before, age 30.
This phenomenon has become more pronounced with the current generation because chess is changing faster than it used to. Ditto for Go.
All the reasons you (and others) mentioned are very valid, however they don't explain the trend that top players become increasingly younger.
I do believe that this has a lot to do with how chess is learned. In the past this was much harder: You had to find a coach, analyze openings by hand and gradually gain experience through many games. All of this took time.
Nowadays, in the time of databases and chess engines, all the information is only a click away and accessible to everyone. Experience gained from playing chess is not all that important anymore.
In addition to the plausible answers already given, I'd venture another psychological aspect that resonates well with Leo Skhrnkv's point about family life and other 'distractions':
Lack of single-mindedness. Generally, when people get older, they also get (to the extent applicable to them) wiser. Outside of competitive sport this is a good thing, but in fields like chess it means that not winning a game is not quite the same disaster for one's self esteem that it used to be, other fields of interest start to seem more important, etc. Younger people feel a stronger need to prove themselves and tend to feel that they can afford to put everything but their chosen field into second place.
Compare to this that Magnus Carlsen famously said in an interview that the only reason in his opinion that John Nunn never became world champion is that Nunn is 'too smart', i.e. too good at and too interested in too many other fields.
Note that all the exciting, major World Chess Championships occurred about 20-30 years ago (Kasparov, Karpov, Anand, Kramnik).
Hence, young people (people who are now in their 30s) got interested in chess.
This occurred especially in Europe and somewhat in US's major cities like NYC.
So, more than mental capacity, it is the history and motivation 20-30 years ago that accounts for large volume of GMs in 30s.
For chess at tournament level, good physical condition is an absolute must. And it is well known that physical condition decreases after an age of about 30 years. Chess players can remain top-level somewhat longer than professional football players (maybe because of a lesser frequency of physical injuries, and because stamina lasts longer than sprinter qualities), but nevertheless they are affected by age. The effect was clearly visible in the World Championship match between Carlsen and Anand: Carlsen played out his superior physical condition against Anand (e.g., by playing out drawn looking endgames and making games longer).
Chess is dominated by young players because other than your brain no part of the body is really involved. True, you need to have a healthy and fit body to concentrate on the game hard, but technically your brain is the only part that is involved.
During your childhood, your memory and intelligence are sharper compared to your youth years. Your learning capability is much more, and your intake for remembering patterns is higher. One more plus as a child is that your focus is not deterred by other factors. Your parents make sure you get more learning, and other responsibilities are taken care by them. You even get more time to learn but when you are in your 20s, then there are various factors which does not allow you to concentrate entirely on chess which makes you sometimes learn more slowly.
To all the factors already listed, I would also add fatigue and, to some extent, boredom.
If you start playing chess at the highest level when you are still in your mid-late teens, by the time you're 40 you've been around for more than 20 years, and that is 20 plus years of being around, traveling for the good part of the year and studying chess many hours every day.
It is very natural that when your intellectual abilities and your capacity of recovering from stress starts fading you may be willing to slow down your pace and that itself has an impact on your playing strength.
