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.