Why do you think often faster coders are younger? With many more tricks up their sleeves, a more experienced coder will be faster. Or are you saying compared to literally older people, but not experienced?
I don't think the assertion has any basis in objective data in the first place but is based on a personal feeling. I don't see the value of trying to discuss reasons for something that only "exists" based on spurious subjective personal claims?
Young coders can code that 10,000 line bad idea they had really fast, while older ones can recognize that a problem isn't new and avoid the 10,000 lines altogether.
Cognitively, people do slow down after 21 or so. It is likely not to be a linear process, but I'm sure a quick google of research will find this. It is not, as suggested by the other comment, spurious feeling, but a well-known cognitive fact about humans. And experience in eal things doens't help with random tests, expecially if you're 20 more years away from your CS finals.
Whether this translates into a perceptible disadvantage in code tests is less likely to have been tested, but it is given what we know, and given other arguments above, quite possible and indeed likely.
At least one study suggests that unlike athletic abilities that do peak at around 20, fluid intelligence is more complex and may peak all the way up to 50 years. http://m.pss.sagepub.com/content/26/4/433