 The theory has often been extended to a critical period for second-language acquisition (SLA), although this is much less widely accepted.Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages.David Singleton states that in learning a second language, "younger = better in the long run", but points out that there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood—long after any critical period has presumably come to a close.