A 10-month-old girl born to non-consanguineous parents presents with bilateral profound sensorineural hearing loss (>90 dB) detected on universal newborn hearing screening. Genetic testing reveals a homozygous Connexin 26 (GJB2) mutation. Imaging shows normal cochlear anatomy with no evidence of ossification. The audiogram demonstrates the pattern marked **D** — cochlear implant candidacy. Which of the following is the MOST critical factor determining optimal post-implant speech and language outcomes in this child?