at about 12, after which learners lose the ability to fully learn a language. However, the exact age marking the end of the critical period is debated, and ranges from age 6 to 13, with many arguing that it is around the onset of puberty. This strict version has since been rejected for second-language acquisition, as some adult and adolescent learners have been observed who reach native-like levels of pronunciation and general fluency faster than young children. However, in general, adolescent and adult learners of a second language rarely achieve the native-like fluency that children who acquire both languages from birth display, despite often progressing faster in the initial stages. This has led to speculation that age is indirectly related to other, more central factors that affect language learning. Children who acquire two languages from birth are called simultaneous bilinguals. In these cases, both languages are spoken to the children by their parents or caregivers and they grow up knowing the two languages. These children generally reach linguistic milestones at the same time as their monolingual peers. Children who do not learn two languages from infancy, but learn one language from birth, and another at some point during childhood, are referred to as sequential bilinguals. People often assume that a sequential bilingual's first language is their most proficient language, but this is not always the case. Over time and experience, a child's second language may become his or her strongest. This is especially likely to happen if a child's first language is a minority language spoken at home, and the child's second language is the majority language learned at school or in the community before the age of five. Proficiency for both simultaneous and sequential bilinguals is dependent upon the child's opportunities to engage in meaningful conversations in a variety of contexts. Often simultaneous bilinguals are more proficient in their languages than sequential bilinguals. One argument for this is that simultaneous bilinguals develop more distinct representations of their languages, especially with regards to phonological and semantic levels of processing. This would cause learners to have more differentiation between the languages, leading them to be able to recognize the subtle differences between the languages that less proficient learners would struggle to recognize. Learning a language earlier in life would help develop these distinct representations of language, as the learner's first language would be less established. Conversely, learning a language later in