We have plasticity but our neocortex has a limited capacity, it’s made up of pattern recognisers - I estimate about 300 million of them. People say we only use 10 percent of our brains, actually we use all of it. It’s just not organised that well.
The reason that people, as they get older, have more difficulty learning things compared to a child, is that a child has all this virgin neocortex, all these pattern recognisers that can be filled up with information. A newborn has twice as many connections as an adult, so it’s been pruned to reflect the knowledge that the person has gained. We have already filled it up with information; there is a process where we can learn new things but we actually have to abandon these patterns.
There’s lot of redundancy, so we can give up some of the redundancy and still remember something, but that’s why memories fade. We do have plasticity but it’s a skill to essentially do “garbage collection” on your neocortex to get rid of patterns that are really no longer of use.