Whether adults gradually lose the capacity to gain knowledge is open to question. There are conflicting theories concerning the ability to reason and process information easily. One suggests that cognition remains substantially the same after reaching adulthood. Another proposes that there is a gradual decrease, beginning in your 20s, in the ability to organize and utilize information, resulting primarily from natural ebbing of intellectual activity. Other hypotheses present the ideas that the ability to learn new things decreases over time but thinking, reasoning and remembering increase with age---not necessarily when it comes to book learning but definitely in the arena of solving real-life problems. Adults may process new information more slowly, but the old stereotype that only young people are smart and old people are mentally slow is simply not factual.
There are many levels of intelligence among adults but, in the absence of neurological impairment, people can learn at any age. In most people, the fully-developed adult brain is capable of demonstrating objectivity in the way it views the world. It can indulge multiple points of view and see numerous possibilities. Although they vary in ability, adults demonstrate analytical thinking and the skill to identify contrasting elements in any given situation. Because memory tends to decline with age, some older adults may have a harder time processing new content, retaining information and recalling it later. This doesn't imply that they are any less smart than they ever were, but rather that intellectual abilities may decrease with age. Adults need more time to learn, but if they can control the pace, they can successfully grasp new things.
For the practical adult thinker, the value of learning anything new is the useful applications it offers. Adults want to know if what is being presented is practical and relevant to their lives. Critical thinking skills, developed over time, have honed adults' ability to tackle the specifics of a problem and render a solution. When an adult is presented with a new problem, his ability to solve it depends on individual accumulated knowledge and skills.
Ben Franklin once said, "Tell me and I forget, teach me and I remember, involve me and I learn." So it is with adult learners. Give them a part in the process, make knowledge relevant to their lives and let them experience it, and they will successfully attach it to what they already know, thus taking ownership of a new skill or understanding. Retention comes not by rote memorization but by relevancy. Every time adults learn, they re-create themselves and the world around them.