Regarding your post, you just reminded me of a brilliant piece by Jean Paul Sartre (cant remember the exact title, but it was something like Isles Flotantes).
anyway, the bottomline of this article was about people that speciaise on a topic, age, and how potential mental senility began (in the topic at question for him at least) around 35 years of age. this assumes the individual has specialised in his field of study, becomes a specialist that practices his discipline, and stops learning.
However, in this article, he also mentioned which "specialists" do not suffer that mental senility. interestingly, it is those that continue learning. but learning from who you may ask? learning from new generations studying to become specialists is one way. this may mean becoming a teacher, but it is not a necessary condition.
i guess the bottomline is that when you think you cant learn any more in your topic of specialisation, then you are faced with this "mental senility" issue.
this is why i disagree with your statement.
j