The importance of practice and review in the learning process proves itself through the actions of people who do or do not choose to carry it out. For example, professional athletes become extremely skilled at the sports they do. Only a fraction of the credit for their success can go to their talents, whereas the majority of their skills come from the countless hours they have spent (usually starting from a young age) practicing and learning about their sports.
The same goes for any mathematician or scientist; they spend countless hours each day practicing their trade. This is what accounts for their knowledge and is also the reason that they continue to learn and improve upon their skills. Any respected scientist, athlete, mathematician, or expert of any kind continually uses their skills, and more importantly, expands upon them.
The truth is that the human brain is not good at remembering things after they become unused and therefore unnecessary. This is why people who continually practice and learn about their skills achieve the highest level of learning. People who start from a young age therefore have the maximum amount of time to learn and expand their skill set. Even if a person does not decide to start a learning process at a young age, that person can always benefit from starting his or her learning process as soon as possible.