Effects of Age at Cochlear Implantation on Vocabulary and Grammar: A Review of the Evidence

Purpose: The increasing prevalence of pediatric cochlear implantation over the past 25 years has left little doubt that resulting improvements in hearing offer significant benefits to language development for many deaf children. Furthermore, given the documented importance of access to language from birth, there has been strong support for providing congenitally deaf children with implants as early as possible. Earliest implantation, in many ways, has become the "gold standard" in pediatric cochlear implantation, on the assumption that it is the key to language development similar to that of hearing children. Empirical evidence to support this assumption, however, appears more equivocal than generally is believed. This article reviews recent research aimed at assessing the impact of age at implantation on vocabulary and grammatical development among young implant users. Method: Articles published between 2003 and 2018 that included age at implantation as a variable of interest and in which it was subjected to statistical analysis were considered. Effect sizes were calculated whenever possible; we conducted a multivariate meta-analysis to compare outcomes in different language domains. Results: Taken together, findings from 49 studies suggest that age at implantation is just one of a host of variables that influence vocabulary and grammatical development, its impact varying with several factors including whether age at implantation is treated as a dichotomous or continuous variable. Results from a meta-analysis showed significant differences across language domains. Conclusion: The pattern of results obtained indicates the importance of considering various child, family, and environmental characteristics in future research aimed at determining how early "early implantation" needs to be and the extent to which age at implantation, duration of implant use, and other factors influence language and language-related outcomes.