The ease of learning a second language depends almost exclusively on environmental factors. Students whose parents enter them in early education and who read to them frequently are more likely to have extra income and more free time. Students who begin studying a language later in life would have had an easier time learning some aspects of that language if they had begun studying it as a young child. Proficient speakers of a second language are likely to have begun learning it before the age of 6.
A study of young children' s ability to learn foreign languages found that those with parents who read them more than one book per week in their native language were 75% more proficient in the foreign languages that they learned than children whose parents read them one book per week or less. A secondary finding of the study was that children' s ability to remember new vocabulary in a second language drops off sharply after the age of 6, when it becomes 75% more difficult to retain new words learned in the second language.
Assuming the statements above are true, which of the following can be inferred from them?