TZK, I advise you read more into neuroplasticity. It's not as if portions of the brain are lost or hit by alpha particles (free radicals). While that does happen, it does not generally result in the all out ability/memory loss you imply.
Basically, we're born with a central nervous system, itself composed of vast connections of neurons. Single neurons branch out and connect with multiple others. The more branches, the more associations. As we age, the substructure gets more and more defined, and new branches then attach to that.
Think of rain falling over decades on the same mountain. When the first rains fell, the little streams created went in all different ways. Then, as the rains continue, certain streams get deeper and wider, and eventually when it rains all water tends to flow into these main tributaries. It's not as if new tributaries cannot branch out, it's just that they tend to be more closely connected to the main.
Older people have deeper "streams" making up their nervous system. They have new experiences all of the time, and continue to make new associations, but they also have the machinery which explains a much greater percentage of all new experience than they had when younger.
They are less neural plastic, but absolutely capable.
Cheers.
