Susan would like for her child to learn a second language. If she wants the child to be fluent in the second language, research suggests that she should begin instruction no later than age __________.

Research suggests that Susan should ideally begin instruction no later than around the age of 7 for her child to have the best chances of achieving fluency in a second language. This age range falls within what is often referred to as the "critical period" for language learning, which extends roughly till puberty.

Extra: The concept of the "critical period" for language acquisition comes from the theory that there is a specific timeframe during which the human brain is exceptionally receptive to language learning. During early childhood and up to puberty, children are believed to be more adept at picking up languages due to the plasticity of their developing brains. This includes more easily acquiring pronunciation, grammar structures, and vocabulary. This doesn't mean that after this period language learning is impossible; adults can also learn new languages, but it may require more effort and the learning process may be different, often relying on more explicit grammar instruction and memorization. For young children, language learning is more natural, and they often learn through immersive environments, repetition, and mimicking the speech patterns they hear. Therefore, Susan's consideration to start early could greatly benefit her child in achieving fluency in a second language.