Synapse overproduction and selection

Initial brain development in animals occurs by overproducing synapses and then pruning away those that aren't used. This allows for maximal flexibility and adaptation to the environment. For example, if one eye is damaged at birth, the other eye will get more synaptic connections, and the damaged eye will receive fewer synaptic connections. In adults, however, new synaptic connections are built as learning occurs, rather than pruning away existing connections. 

The first kind of learning--by pruning away existing connections--enables more complete responses than the second kind. As an analogy, imagine that you are given 1,000 different shades of paint and then start a course in landscape painting. You learn to rely on certain oft-repeated colors. Those less used colors work their way to the bottom of the paint box and may eventually be thrown away altogether. Now, if you suddenly switch to still-lifes or portraiture you may find that you no longer have some colors you need, so you have to make due with less accurate colors. You have exactly what you need for the job you do most often, in this case, painting landscapes. The problem is, once the unused paints are thrown away, you no longer have them if the job changes.

That brings us to the second type of learning; that of building new synaptic connections that were lost long ago, or maybe never existed in the first place. The good news is, it can often be done. For example, an adult speaker of English can, with careful training, be taught to hear sounds in Russian that don't exist in English. Some times. 

Some things may be trained while others may not yield to training. And some things, while trainable, may not ever be mastered as well as if you had used the first way of learning. That's why most people must learn languages before puberty if they want to speak those languages without an accent. You can learn Russian or Chinese or English as an adult, but it's much more difficult to learn to speak it as fluently as you would have had you learned it as a child.