Starting age is not necesarily a predictor of language outcomes, but the quality and quantity of input are.
This study explores the long-term effects of starting age and the effects of input in an instructed language learning setting. First, with respect to the effects of starting age, the findings suggest that in the long term and after similar amounts of input, starting age is not a predictor of language outcomes. Second, the study examines the effects of input using multiple measures derived from responses to an extended questionnaire. The analysis reveals modest but significant effects of input on participants’ proficiency, confirming that input never ceases to play a role in an instructed language learning setting, in contrast with opposite claims from studies of naturalistic language learning.
Munoz, C., 2011. Input and long-term effects of starting age in foreign language learning. International Review of Applied Linguistics in Language teaching, 49, pp. 113-133.