Some people on entering adulthood experience a slowing of their learning to an almost imperceptible rate. I've witnessed this phenomenon enough that I'm moved to try explaining it.
I felt the onset of it in my 21st year. This slowing is natural in the sense that it's both common and socially acceptable. It's a terrible loss for the individual and society. Adults who continue to learn are capable of formidible accomplishments. Those who don't will find themselves in an increasingly binding existence.
In the culture in which I grew up, aging smelled of defeat. My uncle who struggles to use email or a web browser finds himself in a precarious position if he's not constantly surrounded by sympathetic helpers. Blaming the uncle is unproductive. There are many like him. Rather, the blame falls on the society that fails to emphasize the importance of cultivating the mind. This belief that aging is defeat seems to propagate by nature of its defeatism. Viktor Frankl's A Man's Search For Meaning to see that the person for whom hope is absent is the most susceptible to death.
The task of reforming this social ill falls on the individuals who are lucky enough to have the time to change themselves. It's possible to surround oneself with human examples, and in so doing, create a realm of possibility. In aging we become an amalgam of the people who we study and interact with.
The act of learning is defined here narrowly as the accural of tacit knowledge. Tacit knowledge is a set of working assumptions about how one's environment functions. This could be knowing how a colleage behaves, or how to drive a car. All knowledge of this sort is cultivated in an environment. When the colleage dies and cars drive themselves, the associated knowledge is uprooted. Parts of it may still be useful, but as the environment inevitably changes, the assumptions loosen and degrade. When assumptions are left untouched, reality may stumble away. The act of cultivation is releasing stale knowledge, tending existing knowledge, and seeking new knowledge.
Gauging whether knowledge will be relevant to a future environment bears strong likeness to gauging an investment. Those who profit are those who can afford to take calculated risks. In learning, this is the intersection of people with time and people with discipline.
Most adults forget that real learning is hard and often stressful. When children learn hard lessons they are sometimes brought to tears by the intensity of the experience. When I prepare a video of myself speaking Chinese to send to my tutor, the stress of it not being good enough comes into conflict with the stress of failing to send it entirely.
Tutors and peers can provide social pressure. A good learning environement is one that allows this stress to be created in a repeatable way.