The term malleable, as is used in psychology, refers to the process of brain augmentation through "neuroplasticity." It is neuroplasticity that allows children to learn quickly through experiences. The more physical and mental stimulation a child receives during their formative years the more neural pathways they will build that can then build associations with later learning thereby making education faster and easier. This is why there is such an emphasis on early childhood education through preschools, Head Start, and the plethora of educational toys that are sold. On the other hand, as people age, their intellectual capacity seems to deteriorate. This frequently occurs as a result of lack of stimulation, boredom, health problems, or overmedication. However, through activities that require mental focus and problem-solving like handcrafts (knitting and crocheting), puzzle-solving (crosswords and sudoku), and hobbies (golf, woodwork, etc.) can help to fend off the intellectual effects of aging.
What this means is that the brain, through continued intellectual stimulation, can continue to learn and problem solve into old age and not regress into the sort of "faux "dementia that can occur when an older individual is not being mentally stimulated. A third way that mental malleability effects humans is in their "memories." Memory is information that is neurochemically stored in the brain and does not work like a videotape recording. Memories can change, or be manipulated, over time so that what you "remember" may not be wholly accurate. Memories can be planted, invented, or merely change as time passes. Following traumatic events it is possible for a person who is being interviewed to have memories "planted." The brain then stores the events that were suggested and builds a memory around events that never actually happened. It is also possible while in the grip of an intense dream that the brain may imagine something and incorporate it into memory thereby causing a person to remember something that never happened outside of their own mind.
Likewise, the mind can edit memories so that those memories can be faulty. An example, for instance; you are remembering a family reunion. You remember your Uncle Billy playing a practical joke. However, once you think about this on a time line, you realize Uncle Billy couldn't have been there because he died 2 year earlier. In this example you are substituting Uncle Billy for someone else that likes practical jokes. This type of "editing" frequently effects people who have had traumatic experiences. When being interviewed following such an event the interviewee may alter the events and details, not maliciously, but because of the basic faultiness of human memory. This is why the police and lawyers have very little faith in "eyewitness" memories.